Fixed #6445 -- Allow model instances to be used as a default for ForeignKeys

(via a callable). Also updates the documentation of the "default" attribute to
indicate a callable can be used. Thanks, Philipe Raoult.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7331 bcc190cf-cafb-0310-a4f2-bffc1f526a37