Fixed #13949 - Note that readonly fields can't be modified via custom clean...

Fixed #13949 - Note that readonly fields can't be modified via custom clean methods. thanks alk for the suggestion and elbarto and Gabriel Hurley whose language I used in the final patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15060 bcc190cf-cafb-0310-a4f2-bffc1f526a37