[1.0.X] Fixed #9546: GenericRelations inherited from base models no longer...

[1.0.X] Fixed #9546: GenericRelations inherited from base models no longer query using the wrong content type. Backport of r10373 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10374 bcc190cf-cafb-0310-a4f2-bffc1f526a37