Fixed #6155 -- Fixed dumpdata to work with the default model manager (necessary

for the rare cases when the 'objects' manager might not even exist). Based on
a  patch from Michael Trier.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6932 bcc190cf-cafb-0310-a4f2-bffc1f526a37