[feat] Update attach to allow https servers

Author: cloverhap

lib/engine.io.js

@@ -3,6 +3,7 @@
  */
 
 var http = require('http');
+var https = require('https');
 
 /**
  * Invoking the library as a function delegates to attach if the first argument
@@ -20,7 +21,8 @@ var http = require('http');
 exports = module.exports = function () {
   // backwards compatible use as `.attach`
   // if first argument is an http server
-  if (arguments.length && arguments[0] instanceof http.Server) {
+  if (arguments.length && 
+    (arguments[0] instanceof http.Server || arguments[0] instanceof https.Server)) {
     return attach.apply(this, arguments);
   }