Fixed #10738 -- Fixed content type values for deferred and proxy models.

Models with deferred fields, or which are proxying for an existing
model, now return the same ContentType object as the real model they
reflect. Thanks to tomasz.elendt for help with fixing this.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10523 bcc190cf-cafb-0310-a4f2-bffc1f526a37