Fixed #12893 -- Added tests to validate that the right queryset is always used...

Fixed #12893 -- Added tests to validate that the right queryset is always used in model admins. Thanks to mk and Julien Phalip for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15578 bcc190cf-cafb-0310-a4f2-bffc1f526a37