Fixed #7441 -- Removed some of the shortcuts in the doctest output...

Fixed #7441 -- Removed some of the shortcuts in the doctest output comparators, and added a wrapper to allow comparison of xml fragments. Thanks to Leo Soto for the report and fix.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8003 bcc190cf-cafb-0310-a4f2-bffc1f526a37