[fix] Processing error code on abort connection (#562)

Fixes #561

Author: shapel

lib/server.js

@@ -502,7 +502,7 @@ Server.prototype.attach = function (server, options) {
 
 function abortConnection (socket, code) {
   if (socket.writable) {
-    var message = Server.errorMessages.hasOwnProperty(code) ? Server.errorMessages[code] : (code || '');
+    var message = Server.errorMessages.hasOwnProperty(code) ? Server.errorMessages[code] : String(code || '');
     var length = Buffer.byteLength(message);
     socket.write(
       'HTTP/1.1 400 Bad Request\r\n' +