[1.1.X] Fixed #12851 -- Another attempt at fixing select_related() with...

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

Backport of r13059 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13060 bcc190cf-cafb-0310-a4f2-bffc1f526a37