don't add upgrade handler if a custom handler is passed in

if a callback but no static proxy is defined and
no routes are provided then handlers.length is 1.
However the upgrade event is still automagically
attached in spite of having an explicit callback.

lib/node-http-proxy.js

@@ -57,6 +57,7 @@ var HttpProxy    = exports.HttpProxy    = require('./node-http-proxy/http-proxy'
 exports.createServer = function () {
   var args = Array.prototype.slice.call(arguments),
       handlers = [],
+      callback,
       options = {},
       message,
       handler,
@@ -77,7 +78,7 @@ exports.createServer = function () {
       case 'string':   host = arg; break;
       case 'number':   port = arg; break;
       case 'object':   options = arg || {}; break;
-      case 'function': handlers.push(arg); break;
+      case 'function': callback = arg; handlers.push(callback); break;
     };
   });
 
@@ -180,7 +181,7 @@ exports.createServer = function () {
     proxy.close();
   });
 
-  if (handlers.length <= 1) {
+  if (!callback) {
     //
     // If an explicit callback has not been supplied then
     // automagically proxy the request using the `HttpProxy`