Fix: KeyError: 'source' when parsing HTML5 videos

This fixes a bug that causes the video XBlock parsing to break in the new XBlock runtime if:
* the html5_sources field is set, and
* the download_video field is not set

The error is:

  File "/edx/app/edxapp/edx-platform/openedx/core/djangoapps/xblock/rest_api/views.py", line 52, in render_block_view
    block = load_block(usage_key, request.user)
  File "/edx/app/edxapp/edx-platform/openedx/core/djangoapps/xblock/api.py", line 84, in load_block
    return runtime.get_block(usage_key)
  File "/edx/app/edxapp/edx-platform/openedx/core/djangoapps/xblock/runtime/blockstore_runtime.py", line 70, in get_block
    block = block_class.parse_xml_new_runtime(xml_node, runtime=self, keys=keys)
  File "/edx/app/edxapp/edx-platform/common/lib/xmodule/xmodule/video_module/video_module.py", line 620, in parse_xml_new_runtime
    setattr(video_block, key, cls.fields[key].from_json(val))
  KeyError: 'source'

The reason for the error is that parse_video_xml() will sometimes return a 'source' attribute in its field_data return value, even though source is not a video field (anymore). This then causes an error when trying to look up cls.fields['source']

A workaround in the meantime is to add download_video="false" to the OLX.