Tweaked the dummy database backend initialisation.

In order to report more comphrehensible tracebacks, remove the super()
constructor call, since passing args to object.__init__ ends badly. So some
subclassing possibilities are now removed, but it's the "dummy" backend, so
we can make some compromises.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10042 bcc190cf-cafb-0310-a4f2-bffc1f526a37