avoid unnecessary attempt to update an immutable dict

request.data has become immutable under Django 1.11 for some reason.
Regardless, setting the 'id' key should be extraneous because we already
instruct the update method to use a specific pk (see kwargs) which
should be sufficient to locate the database record and update it.

lms/djangoapps/experiments/views.py

@@ -46,7 +46,6 @@ class ExperimentDataViewSet(viewsets.ModelViewSet):
             try:
                 obj = self.get_queryset().get(user=self.request.user, experiment_id=experiment_id, key=key)
                 self.kwargs['pk'] = obj.pk
-                self.request.data['id'] = obj.pk
                 return self.update(request, *args, **kwargs)
             except ExperimentData.DoesNotExist:
                 pass