title: Initializing GridPanel and ArrayStore with empty array does not work
body: <p>When I initialize my ArrayStore with some data then I can insert new records and everything displays correctly. However, if I initialize the ArrayStore with an empty array then while the first record inserted works every subsequent insert overwrites the first record in the grid.
  This seems a simple problem but I can't find a working way to initialize the grid as empty.</p>
  
  <p>So what am I doing wrong?</p>
  
  <p>Many thanks in advance!</p>
  
  <pre><code>var myData = [
      // Initializing with the following element works.
      //['Hyannis Connector',3456.9,'Hyannis Connector Bridle Path','Hyannis Connector/Baden Powell'],
      // Initializing with this element works but leaves an empty row in the grid..
      //[]
  ];
  
  // create the data store
  segmentStore = new Ext.data.ArrayStore({
      fields: [
         {name: 'trailName'},
         {name: 'segmentLength', type: 'float'},
         {name: 'startJunction'},
         {name: 'endJunction'},
      ]
  });
  
  // manually load local data
  segmentStore.loadData(myData);
  
  // create the Grid
  segmentGrid = new Ext.grid.GridPanel({
      region: 'south',
      store: segmentStore,
      columns: [
          {header: 'Trail Name', width: 140, sortable: true, dataIndex: 'trailName'},
          {header: 'Segment Length', width: 75, sortable: true, dataIndex: 'segmentLength'},
          {header: 'Start Junction', width: 75, sortable: true, dataIndex: 'startJunction'},
          {header: 'End Junction', width: 75, sortable: true, dataIndex: 'endJunction'},            
      ],
      stripeRows: true,
      height: 150,
      width: 600,
      title: 'Trail Segments in Route',
      layout: 'fit'      
  });
  
  var defaultData = {
              trailName: segment.getTrailName(),
              segmentLength: segment.getLength(),
              startJunction: '',
              endJunction: ''
          };
          var recId = 3; 
          var p = new segmentStore.recordType(defaultData, recId); 
          segmentStore.insert(0, p);
  </code></pre>