[fix] do not use middleware code if it's not needed

lib/node-http-proxy.js

@@ -166,7 +166,7 @@ exports.createServer = function () {
     switch (typeof arg) {
       case 'string': host = arg; break;
       case 'number': port = arg; break;
-      case 'function': middleware.push(arg); break;
+      case 'function': middleware.push(handler = callback = arg); break;
       case 'object': options = arg; break;
     };
   });
@@ -203,11 +203,15 @@ exports.createServer = function () {
     }
   }
 
-  if (middleware.length) {
+  if (middleware.length /*> 1*/) {
     //handler = callback = middleware.shift()
     //else if (middleware.length)
     handler = callback = stack(middleware, proxy);
   }
+  else if (middleware.length) { //do not use middleware code if it's not needed.
+    var h = middleware[0]
+    handler = callback = function (req,res) { h(req,res,proxy) };
+  }
 
   if (!handler) {
     //