Fixed #13679, #13231, #7287 -- Ensured that models that have...

Fixed #13679, #13231, #7287 -- Ensured that models that have ForeignKeys/ManyToManyField can use a a callable default that returns a model instance/queryset. #13679 was a regression in behavior; the other two tickets are pleasant side effects. Thanks to 3point2 for the report.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13577 bcc190cf-cafb-0310-a4f2-bffc1f526a37