[1.1.X] Fixed #11940 - ModelForm evaluates callable default values on form class creation

  
Thanks to Harm Geerts for the report and initial patch.

Backport of [12721] from trunk


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12722 bcc190cf-cafb-0310-a4f2-bffc1f526a37