[1.0.X] Fixed #9308 -- Corrected the updated of nullable foreign key fields...

[1.0.X] 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.

Merge of r10822 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@10823 bcc190cf-cafb-0310-a4f2-bffc1f526a37