Fixed #22992 -- regression in .filter(generic_fk=...) error message

Generic Foreign Keys can't be used as lhs in lookups for historical
reasons. Django 1.6 gave a FieldDoesNotExist exception when using GFKs
as lhs in lookups, but due to regression caused by lookup refactor patch
(20bab2cf) the exception type was
changed to AttributeError.

It might be a good idea to add support for gfk__exact and gfk__in lookups
later on.

Thanks to glicerinu@gmail.com for the report. The code in this commit
was written by Tim Graham.

tests/generic_relations/tests.py

@@ -355,6 +355,10 @@ class GenericRelationsTests(TestCase):
         self.assertFalse(created)
         self.assertEqual(tag.content_object.id, diamond.id)
 
+    def test_query_content_type(self):
+        with six.assertRaisesRegex(self, FieldError, "^Cannot resolve keyword 'content_object' into field."):
+            TaggedItem.objects.get(content_object='')
+
 
 class CustomWidget(forms.TextInput):
     pass