Add res.sendStatus method

A helpful method for apps that send many simple status code responses, e.g., REST APIs. The body of the response is set using node's http.STATUS_CODES to avoid problems with clients that choke on empty body responses.
> I'm not suggesting you do this, but we didn't prevent before, so I don't think preventing it now is going to help anyone.

If you're going to add API, I recommend doing it correctly from the start.