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.
Also, `req.is` would always be false for all `GET` requests since they have no body and that's what `req.is` does--determines what type the _request_ is, not what type of response the client is looking for.