title: TreeGrid: setting data root has no effect
body: <p>I'm trying to setup a TreeGrid, my data object looks like this:</p>
  
  <pre><code>{
      "code": "success",
      "data": {
          "text": ".",
          "children": [
              {
                  "clientname": "Market",
                  "contact": "OpenX Market Advertiser",
                  "email": "asdasd@asdasd222.de",
  </code></pre>
  
  <p>I need to tell Ext that it should use <em>data</em> as the root element:</p>
  
  <pre><code>var store = Ext.create('Ext.data.TreeStore', {
      model: 'Task',
      proxy: {
          type: 'ajax',
          url: 'http://localhost/rocketads/trunk/advertisers/index/stats:true/',
          reader:{
              type:'json',
              root:'data'
          }
      },
  });
  </code></pre>
  
  <p>This doesn't work for me although I'm successfully using this with normal Stores.</p>