[1.1.X] Fixed #12851 -- Corrected the loading of values when select_related()...

[1.1.X] Fixed #12851 -- Corrected the loading of values when select_related() is used on inherited models. Thanks to phxx for the report and test case.

Backport of r13054 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13055 bcc190cf-cafb-0310-a4f2-bffc1f526a37