Fixed #23300 -- TestCase.assertTemplateUsed passes erroneously on an HttpResponse

Thanks zags for the report and bmispelon for the patch
I think this error message isn't very helpful (i.e. it's not actionable), and it leaks an internal detail, that we monkey-patch response objects with a `templates` attribute. What happened is a detail, what we need to know is **why** it happened, so you can fix it.