Fixed bug in the executor preventing usage of update and remove without callback

lib/datastore.js

@@ -590,5 +590,4 @@ Datastore.prototype.remove = function () {
 
 
 
-
 module.exports = Datastore;

lib/executor.js

@@ -24,7 +24,8 @@ function Executor () {
         cb();
       };
 
-      task.arguments.push(callback);
+      // task.arguments is an array-like object so push is not available
+      task.arguments[task.arguments.length] = callback;
     }
 
     task.fn.apply(task.this, task.arguments);