Fixed #6886: Tightened up ForeignKey and OneToOne field assignment. Specifically:

  * Raise a ValueError if you try to assign the wrong type of object.
  * Raise a ValueError if you try to assign None to a field not specified with null=True.
  * Cache the set value at set time instead of just at lookup time.

This is a slightly backwards-incompatible change; see BackwardsIncompatibleChanges for more details.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7574 bcc190cf-cafb-0310-a4f2-bffc1f526a37