title: Subscribe callback triggers twice when use computed with notify and rateLimit extender
body: <p>When I use computed notify='always' and rateLimit extender, subscribe callback triggers twice.</p>
  
  <pre><code>    ko.changedFlag = function(root) {
      var result = function() {};
      var initialState = ko.observable(ko.toJSON(root));
  
      result.isChanged = ko.computed(function() {
          var changed = initialState() !== ko.toJSON(root);
          if (changed) result.reset();
          return changed;
      }).extend({ notify: 'always',rateLimit:500 });
  
      result.reset = function() {
          initialState(ko.toJSON(root));
      };
  
      return result;
  };
  // add changed flag property to the model
  model.changedFlag = new ko.changedFlag(model);
  // subscribe to changes
  model.changedFlag.isChanged.subscribe(function(isChanged) {
      if (isChanged)  alert("model changed");
  });
  </code></pre>
  
  <p>Example: <a href="http://jsfiddle.net/GQgpX/39/" rel="nofollow">http://jsfiddle.net/GQgpX/39/</a></p>
  
  <ul>
  <li>Press "add" key or type something to textbox and see result</li>
  </ul>