Ensure context is passed through when calling get_serializer

[jamesfneyer]

When calling get_serializer on ConfigurableGenericViewSet, ensure that context is passed through, and not overwritten by the get_serializer_context in the default get_serializer.

[mlclay]

Please use the same logic the upstream repository used to handle this new functionality: encode/django-rest-framework#7298

[jamesfneyer]

I originally did it this way so that the default context would not be overwritten, but instead expanded.

[mlclay]

LGTM

Thanks for the update for context precedence parity with upstream. I'd like this to not deviate too far from the expected DRF ViewSet functionality where we can manage.