Fixed #8070 -- Cache related objects passed to Model init as keyword arguments. Also:

 * Model init no longer performs a database query to refetch the related objects it is passed.
 * Model init now caches unsaved related objects correctly, too. (Previously, accessing the field would raise `DoesNotExist` error for `null=False` fields.)
 * Added tests for assigning `None` to `null=True` `ForeignKey` fields (refs #6886).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8185 bcc190cf-cafb-0310-a4f2-bffc1f526a37