GenericAPIView now applies filter_backend for list and retrieve api views

Before this commit only the MultipleObjectAPIView would apply a filter_backend, leaving the SingleObjectAPIView to return objects you might otherwise expect to have been filtered out.

It's worth mentioning that when a SingleObjectAPIView makes a request for an object that should be excluded, a 404 is the expected result.

I asked Tom about this in a previous issue (still open at this time)

For reference https://github.com/tomchristie/django-rest-framework/issues/682