Fixed #12851 -- Another attempt at fixing select_related() with inherited...

Fixed #12851 -- Another attempt at fixing select_related() with inherited models, this time with only(). Thanks to phxx for the test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13059 bcc190cf-cafb-0310-a4f2-bffc1f526a37