Fixed #12596. Calling super from a ModelForm's clean method is once again...

Fixed #12596. Calling super from a ModelForm's clean method is once again optional. Failing to call super only skips unique validation as documented. Thanks for the initial patch and tests, carljm.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12269 bcc190cf-cafb-0310-a4f2-bffc1f526a37