Fixed #4714 -- Modified serializers to handle None primary keys correctly....

Fixed #4714 -- Modified serializers to handle None primary keys correctly. This slightly changes the output format for primary keys on JSON and YAML serializers (PKs will be output as 1, rather than "1". However, fixtures in the old format will still load ok. Thanks for the patch, pigletto.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6264 bcc190cf-cafb-0310-a4f2-bffc1f526a37