Fixed #11311 -- Reverted [10952], Refs #10785. Changeset [10952] caused problems with m2m relations between models that had non-integer primary keys. Thanks to Ronny for the report and test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11007 bcc190cf-cafb-0310-a4f2-bffc1f526a37