Set default filter_class if not specified

[sebitoelcheater]

If someone want use just a default ModelFilterset and don't create a class for that, should do it. Please add this code.

@@ -6,7 +6,9 @@ class MongoFilterBackend(BaseFilterBackend):
         filter_class = getattr(view,'filter_class', None)
 
         if filter_class is None:
-            return queryset
+            filter_class = ModelFilterset
+            filter_class.Meta = type("Meta", (object,), {})
+            filter_class.Meta.model = view.serializer_class.Meta.model