Make notifyXml return a Buffer object

If (for example) request.body contains non-ascii characters, this caused the http request to not send the correct `Content-Length` header, which in turn causes errbit to not read the entire document.

I changed `Airbrake.prototype.notifyXml` so that it returns a `Buffer` object, which will report a proper length when asked for it in `Airbarke.prototype.notify`.