Fixed #18086 -- Restored '-pk' as the default order in the admin changelist....

Fixed #18086 -- Restored '-pk' as the default order in the admin changelist. This rectifies a slight change in behavior introduced in Django 1.4 and r17635.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17881 bcc190cf-cafb-0310-a4f2-bffc1f526a37