Fixed #11905: Raise an error on model form creation if a non-existent field...

Fixed #11905: Raise an error on model form creation if a non-existent field was listed in fields. Thanks ben and copelco.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@13739 bcc190cf-cafb-0310-a4f2-bffc1f526a37