title: Backbone.js Model previous method not working?
body: <p>Strangely I noticed that the model previous method is not working the way I thought.. it keeps returning the same value as the get. I think that something is wrong with my code or backbone.js is not upgrading the this._previousAttributes when change event is fired.</p>
  
  <pre><code>model = new Backbone.Model()
  
  model.set({attr1: 123})
  
  alert(model.previous("attr1"))  //alert 123 instead of undefined
  alert(model.get("attr1"))
  
  model.set({attr1: 312})
  
  alert(model.previous("attr1"))  //alert 321 instead of 123
  alert(model.get("attr1"))
  </code></pre>
  
  <p><a href="http://jsfiddle.net/wLKBk/" rel="nofollow">http://jsfiddle.net/wLKBk/</a></p>
  
  <p>What am I doing wrong?</p>