Fix #17876: Corrected an exception (regression) raised where select_realted...

Fix #17876: Corrected an exception (regression) raised where select_realted and only is used on a proxy model. Thanks milosu and charettes.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@17692 bcc190cf-cafb-0310-a4f2-bffc1f526a37