Fixes uncaught errors when Response() is created with certain status strings.

Fix errors when Response is called with certain status strings.

If `Response()` is called with a status string that contains only a status code (e.g. `Response('foo', status="404")`) or only a reason (e.g. `Response('foo', status="bar baz")`, the application will crash because status code and message can't be parsed properly.

It could happen for example, if you create a `Response` based on a request made to another site. (which can in theory omit the reason in its status line)