prevent return "undefined" immediately when key is "constructor" for memoize method.

Author: dacoozheng

lib/async.js

@@ -1089,12 +1089,12 @@
         var memoized = _restParam(function memoized(args) {
             var callback = args.pop();
             var key = hasher.apply(null, args);
-            if (key in memo) {
+            if (memo.hasOwnProperty(key)) {
                 async.setImmediate(function () {
                     callback.apply(null, memo[key]);
                 });
             }
-            else if (key in queues) {
+            else if (queues.hasOwnProperty(key)) {
                 queues[key].push(callback);
             }
             else {