Fixed #10785 -- Corrected a case for foreign key lookup where the related...

Fixed #10785 -- Corrected a case for foreign key lookup where the related object is a custom primary key. Thanks to Alex Gaynor for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10952 bcc190cf-cafb-0310-a4f2-bffc1f526a37