Fixed #16848 - Adjusted SimpleTemplateResponse.__init__ to be less brittle.

Could have reverted r16830 instead, but HttpResponse shouldn't have to dance
around and do non-obvious things to keep TemplateResponse happy,
TemplateResponse should be robust against the possibility that
HttpResponse.__init__ might set self.content.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16831 bcc190cf-cafb-0310-a4f2-bffc1f526a37