[1.2.X] Fixed #11486 -- Corrected the XML serializer to allow for the...

[1.2.X] 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.

Backport of r13862 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13863 bcc190cf-cafb-0310-a4f2-bffc1f526a37