[1.0.X] Fixed #9969: choices with options groups (added in [7977]) now work...

[1.0.X] Fixed #9969: choices with options groups (added in [7977]) now work correctly in the admin with list_display and list_filter. Thanks, ramiro. Backport of r10318 from trunk; thanks, cramm.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10340 bcc190cf-cafb-0310-a4f2-bffc1f526a37