Make get_queryset work with django-filter

DjangoFilterBackend would raise `issubclass() arg 2 must be a class or tuple of classes` when only specifying `serializer_class` and `get_queryset`. Example:

```python
class AccountListView(generics.RetrieveAPIView):
    serializer_class = MyModelSerializer
    filter_class = MyDjangoFilter
    filter_backend = DjangoFilterBackend

    def get_queryset(self):
        return MyModel.objects.filter(some_field='foo')
```