DRF 3.7.4 changed how you delegate to another view, so don't

The error in the test was:
```
AssertionError: The `request` argument must be an instance of `django.http.HttpRequest`, not `rest_framework.request.Request`.
```
The (controversial) incompatible change was in 3.7.4: https://github.com/encode/django-rest-framework/pull/5618

I'll look into whether there's another way to address it.

<details>
  <summary>Full error report</summary>

```
AssertionError: The `request` argument must be an instance of `django.http.HttpRequest`, not `rest_framework.request.Request`.

Stacktrace

self = <lms.djangoapps.discussion_api.tests.test_views.CommentViewSetListTest testMethod=test_profile_image_request_for_null_endorsed_by>

    def test_profile_image_request_for_null_endorsed_by(self):
        """
        Tests if 'endorsed' is True but 'endorsed_by' is null, the api does not crash.
        This is the case for some old/stale data in prod/stage environments.
        """
        self.register_get_user_response(self.user)
        thread = self.make_minimal_cs_thread({
            "thread_type": "question",
            "endorsed_responses": [make_minimal_cs_comment({
                "id": "endorsed_comment",
                "user_id": self.user.id,
                "username": self.user.username,
                "endorsed": True,
            })],
            "non_endorsed_resp_total": 0,
        })
        self.register_get_thread_response(thread)
        self.create_profile_image(self.user, get_profile_image_storage())

        response = self.client.get(self.url, {
            "thread_id": thread["id"],
            "endorsed": True,
>           "requested_fields": "profile_image",
        })

lms/djangoapps/discussion_api/tests/test_views.py:1446:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:291: in get
    response = super(APIClient, self).get(path, data=data, **extra)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:208: in get
    return self.generic('GET', path, **r)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:237: in generic
    method, path, data, content_type, secure, **extra)
../venvs/edxapp/local/lib/python2.7/site-packages/django/test/client.py:416: in generic
    return self.request(**r)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:288: in request
    return super(APIClient, self).request(**kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/test.py:240: in request
    request = super(APIRequestFactory, self).request(**kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/django/test/client.py:501: in request
    six.reraise(*exc_info)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/exception.py:41: in inner
    response = get_response(request)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py:249: in _legacy_get_response
    response = self._get_response(request)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py:187: in _get_response
    response = self.process_exception_by_middleware(e, request)
../venvs/edxapp/local/lib/python2.7/site-packages/django/core/handlers/base.py:185: in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/django/utils/decorators.py:185: in inner
    return func(*args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/django/views/decorators/csrf.py:58: in wrapped_view
    return view_func(*args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/viewsets.py:95: in view
    return self.dispatch(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:494: in dispatch
    response = self.handle_exception(exc)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:491: in dispatch
    response = handler(request, *args, **kwargs)
lms/djangoapps/discussion_api/views.py:505: in list
    form.cleaned_data["requested_fields"],
lms/djangoapps/discussion_api/api.py:659: in get_comment_list
    results = _serialize_discussion_entities(request, context, responses, requested_fields, DiscussionEntity.comment)
lms/djangoapps/discussion_api/api.py:468: in _serialize_discussion_entities
    request, results, usernames, discussion_entity_type, include_profile_image
lms/djangoapps/discussion_api/api.py:413: in _add_additional_response_fields
    username_profile_dict = _get_user_profile_dict(request, usernames=','.join(usernames))
lms/djangoapps/discussion_api/api.py:350: in _get_user_profile_dict
    user_profile_details = AccountViewSet.as_view({'get': 'list'})(request).data
../venvs/edxapp/local/lib/python2.7/site-packages/django/views/decorators/csrf.py:58: in wrapped_view
    return view_func(*args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/viewsets.py:95: in view
    return self.dispatch(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:477: in dispatch
    request = self.initialize_request(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/viewsets.py:118: in initialize_request
    request = super(ViewSetMixin, self).initialize_request(request, *args, **kwargs)
../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/views.py:381: in initialize_request
    parser_context=parser_context
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <rest_framework.request.Request object at 0x7f597c773890>
request = <rest_framework.request.Request object at 0x7f597fa20f90>
parsers = [<openedx.core.lib.api.parsers.MergePatchParser object at 0x7f59810c32d0>]
authenticators = [<edx_rest_framework_extensions.auth.jwt.authentication.JwtAuthentication object at 0x7f597c713690>, <openedx.core.lib...rest_framework_extensions.auth.session.authentication.SessionAuthenticationAllowInactiveUser object at 0x7f597c773390>]
negotiator = <rest_framework.negotiation.DefaultContentNegotiation object at 0x7f597fa20410>
parser_context = {'args': (), 'kwargs': {}, 'view': <openedx.core.djangoapps.user_api.accounts.views.AccountViewSet object at 0x7f597c7846d0>}

    def __init__(self, request, parsers=None, authenticators=None,
                 negotiator=None, parser_context=None):
        assert isinstance(request, HttpRequest), (
            'The `request` argument must be an instance of '
            '`django.http.HttpRequest`, not `{}.{}`.'
>           .format(request.__class__.__module__, request.__class__.__name__)
        )
E       AssertionError: The `request` argument must be an instance of `django.http.HttpRequest`, not `rest_framework.request.Request`.

../venvs/edxapp/local/lib/python2.7/site-packages/rest_framework/request.py:159: AssertionError
```
</details>

lms/djangoapps/discussion/rest_api/api.py

@@ -61,6 +61,7 @@ from openedx.core.djangoapps.django_comment_common.signals import (
     thread_voted
 )
 from openedx.core.djangoapps.django_comment_common.utils import get_course_discussion_settings
+from openedx.core.djangoapps.user_api.accounts.api import get_account_settings
 from openedx.core.djangoapps.user_api.accounts.views import AccountViewSet
 from openedx.core.lib.exceptions import CourseNotFoundError, DiscussionNotFoundError, PageNotFoundError
 
@@ -365,10 +366,11 @@ def _get_user_profile_dict(request, usernames):
 
         A dict with username as key and user profile details as value.
     """
-    request.GET = request.GET.copy()  # Make a mutable copy of the GET parameters.
-    request.GET['username'] = usernames
-    user_profile_details = AccountViewSet.as_view({'get': 'list'})(request).data
-
+    if usernames:
+        username_list = usernames.split(",")
+    else:
+        username_list = []
+    user_profile_details = get_account_settings(request, username_list)
     return {user['username']: user for user in user_profile_details}