Fixed #11418 -- formset.cleaned_data no longer raises AttributeError when is_valid is True. Thanks mlavin!

This also introduces a slightly backwards-incompatible change in
FormSet's behavior, see the release docs for details.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14667 bcc190cf-cafb-0310-a4f2-bffc1f526a37