Fixed #11486 -- Corrected the XML serializer to allow for the serialization of objects with a null PK value. Also includes migration of doctests to unittests (we have always been at war with doctests). Thanks to zdmytriv for the report, and Niall Kelly for the patch.

      git-svn-id: http://code.djangoproject.com/svn/django/trunk@13862 bcc190cf-cafb-0310-a4f2-bffc1f526a37