Fixed #14880 - raw_id_fields in admin does not work when limit_choices_to...

Fixed #14880 - raw_id_fields in admin does not work when limit_choices_to dictionary has value=False

Thanks to smallming for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15348 bcc190cf-cafb-0310-a4f2-bffc1f526a37