Emit 'close' in OutgoingResponse and IncomingResponse when the underlying stream is closed before finishing

Also add a 'cancel' method to IncomingResponse which simply resets the stream.
I'd move this from IncomingResponse to IncomingMessage as it applies to IncomingRequests too (see the [built-in http module manual](http://nodejs.org/api/http.html#http_event_close_2))