Fixed #8291 -- Allowed 'pk' to be used as an ordering option in `Model.Meta`....

Fixed #8291 -- Allowed 'pk' to be used as an ordering option in `Model.Meta`. Thanks to peterd12 for the report and to evan_schulz, gruszczy, frog32 and David Gouldin for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17445 bcc190cf-cafb-0310-a4f2-bffc1f526a37