[1.2.X] Fixed #14823 -- Corrected bootstrapping problems with...

[1.2.X] Fixed #14823 -- Corrected bootstrapping problems with register_serializers. Thanks to miker985@uw.edu for the report and draft patch.

Backport of r15336 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15339 bcc190cf-cafb-0310-a4f2-bffc1f526a37