Fixed #9308 -- Corrected the updated of nullable foreign key fields when...

Fixed #9308 -- Corrected the updated of nullable foreign key fields when deleting objects. Thanks to Bob Thomas for the fix, and markshep for the improvements on the test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10822 bcc190cf-cafb-0310-a4f2-bffc1f526a37