Use the get_queryset from a resource if available

If a resource overrides the default get_queryset(), then
ModelMixin-based classes should use it instead of returning all objects
of the model of the resource.

The new order of returning a queryset is:
1. self._resource.get_queryset()
2. self.resource.queryset
3. self.resource.model.objects.all()

djangorestframework/mixins.py

@@ -513,7 +513,10 @@ class ModelMixin(object):
         """
         Return the queryset for this view.
         """
-        return getattr(self.resource, 'queryset',
+        if hasattr(self._resource, 'get_queryset'):
+            return self._resource.get_queryset()
+        else:
+            return getattr(self._resource, 'queryset',
                            self.resource.model.objects.all())
 
     def get_ordering(self):