Ensure that custom primary keys are always shown on a formset. Refs #7938....

Ensure that custom primary keys are always shown on a formset. Refs #7938. Thanks magneto for discovering this problem.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8179 bcc190cf-cafb-0310-a4f2-bffc1f526a37