Fixed #15247 -- Ensured that if a SingleObject view defines get_object but not get_queryset, the ModelFormMixin doesn't fail. Thanks to Sergey N. Belinsky for the report and test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15540 bcc190cf-cafb-0310-a4f2-bffc1f526a37