title: Sencha touch 2 filterby() not updating records
body: <p>I have a nested list on one of the pages of a Tabbed Panel app that is pulling data from "offices.json"</p>
  
  <p>I should like to be able to filter this list when a user clicks on a toolbar button. However my filterBy() function doesn't update the store and the list of offices I can see, even though I can see in the console it is iterating the records and finding a match. What am I doing wrong?
  (And yes I have tried doing s.load() both before and after the filterBy to no avail!)</p>
  
  <pre class="lang-js prettyprint-override"><code>toolbar:{                        
     items:[{
             text: 'Near you',
             id: 'btnNearYou',
             xtype: 'button',
             handler: function() {
               s = Ext.StoreMgr.get('offices');
               s._proxy._url = 'officesFLAT.json';        
               console.log("trying to filter");
               s.filterBy(function(record) {
                var search = new RegExp("Altrincham", 'i'); 
                if(record.get('text').match(search)){
                 console.log("did Match");
                 return true;
                }else {
                 console.log("didnt Match");
                 return false;
               }
             });
             s.load();
            }                            
     }]
  </code></pre>
  
  <p>For the record I'm defining my store like so: </p>
  
  <pre class="lang-js prettyprint-override"><code>store: {
      type: 'tree',
      model: 'ListItem',
      id: 'offices',
      defaultRootProperty: 'items',
      proxy: {
          type: 'ajax',
          root: {},
          url: 'offices.json',
          reader: {
              type: 'json',
              rootProperty: 'items'
          }
      }
  }
  </code></pre>