Fixed #13004: Ensure the add page for a model with a ManyToManyField specified

      in readonly_fields does not raise an exception. Thanks hejsan, mlavin, copelco.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@12827 bcc190cf-cafb-0310-a4f2-bffc1f526a37