Added a test from Jamie Gennis to ensure #9848 doesn't reappear.

The bug itself was fixed at some point in the past months (there have
been a few improvements to update() recently). Fixed #9848.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10528 bcc190cf-cafb-0310-a4f2-bffc1f526a37