Fixed #7913 -- Corrected backwards incompatible parts of [7977] when optgroup...

Fixed #7913 -- Corrected backwards incompatible parts of [7977] when optgroup handling was added to field choices (Ticket #4412). Thanks to Michael Elsdorfer (miracle2k) for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8102 bcc190cf-cafb-0310-a4f2-bffc1f526a37