[api] Expose adapted version of `stack` so it can be used with HttpProxy...

[api] Expose adapted version of `stack` so it can be used with HttpProxy instances not created by `httpProxy.createServer()`

lib/node-http-proxy.js

@@ -115,10 +115,10 @@ exports.setMaxSockets = function (value) {
 };
 
 //
-// stack
+// ### function stack (middlewares, proxy)
 // adapted from https://github.com/creationix/stack
 //
-function stack (middlewares, proxy) {
+exports.stack = function stack (middlewares, proxy) {
   var handle;
   middlewares.reverse().forEach(function (layer) {
     var child = handle;
@@ -206,7 +206,7 @@ exports.createServer = function () {
   }
 
   if (middleware.length > 1) {
-    handler = callback = stack(middleware, proxy);
+    handler = callback = exports.stack(middleware, proxy);
   }
   else if (middleware.length) { 
     //