Fixed #9023 -- Corrected a problem where cached attribute values would cause a...

Fixed #9023 -- Corrected a problem where cached attribute values would cause a delete to cascade to a related object even when the relationship had been set to None. Thanks to TheShark for the report and test case, and to juriejan and Jacob for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11009 bcc190cf-cafb-0310-a4f2-bffc1f526a37