title: Angularjs orderBy in ng-repeat not working as expected
body: <p>I have the following div in my template:</p>
  
  <pre><code>&lt;div ng-repeat="item in billing_history | orderBy:'-timestamp'"&gt;{{ item.date }}&lt;/div&gt;
  </code></pre>
  
  <hr>
  
  <p><code>console.log(JSON.stringify($scope.billing_history))</code> gives me the following:</p>
  
  <pre><code>{
    "May, 2015":{
       "date":"May, 2015",
       "timestamp":1432921230
    },
    "March, 2015":{
       "date":"March, 2015",
       "timestamp":1427846400
    },
    "February, 2015":{
       "date":"February, 2015",
       "timestamp":1425168000
    }
  }
  </code></pre>
  
  <hr>
  
  <p>No matter what, this is what is displayed:</p>
  
  <pre><code>February, 2015
  March, 2015
  May, 2015
  </code></pre>
  
  <p>I've tried <code>orderBy:'-timestamp'</code> and <code>orderBy:'+timestamp'</code></p>
  
  <p>I'm really not sure why this isn't working. Does anyone see anything that could be going wrong?</p>