title: can't group in grid extjs 3.4
body: <p>I have made a webapp with extjs 3.4.
  I have a grid that I want to group with field <code>street</code> but I can't.
  My code is here: </p>
  
  <pre><code>var store_intervencoes =  new Ext.data.GroupingStore({
                  reader: new Ext.data.JsonReader({
                  root:'rows',
                  autoLoad: false,
                  type : 'ajax',
                  //remoteGroup: false,
                  groupField: 'street',
                  fields:[{ name:'street', type:'integer'},
                      { name:'intervenction', type:'string'},
                      { name:'priority', type:'integer'},
                      { name:'value', type:'string'}]
                  })
          }); 
  
  
  var gridPanel = new Ext.grid.GridPanel({
              header: false,
              store: store_intervencoes,
              selModel: new Ext.grid.RowSelectionModel({singleSelect:false}),
              layout: 'fit',
              height: 300,
              width: 1000,
              columnLines: true, 
              cls: 'grid_intervencoes',
              autoScroll: true,
              stripeRows: true,
              autoFill : true,                        
              viewConfig: { emptyText: 'nothing', deferEmptyText: false },
              groupField: 'street',
              view: new Ext.grid.GroupingView({
                          forceFit:true,
                          autoHeight: true,
                          autoFill : true,
                          groupTextTpl: '{text} ({[values.rs.length]} {[values.rs.length &gt; 1 ? "Items" : "fk_n_rua"]})'
                                          }),
              columns: [{
                  //header: "Nome",
                  //sortable: true,
                  dataIndex: "intervenction",
                  //width: 450
                  },{
                  //header: "Prioridade",
                  //sortable: true,
                  dataIndex: "priority",
                  //width: 50
                  },{
                  //header: "Custo" + ' \u20AC',
                  //sortable: true,
                  dataIndex: "value",
                  //width: 50
                  },{
                  //header: 'Trecho',
                  //sortable: true,
                  dataIndex: "street",
                  //width: 50,
                  //groupable: true
                  }],
                  collapsible: true,
                  animCollapse: false,
                  title: 'Grouping Example',
                  iconCls: 'icon-grid',
                  fbar  : [{
                      xtype: 'numberfield',
                      id: 'total',
                      name: 'total',
                      fieldLabel: 'Total',
                      value: '0',
                      readOnly: true,
                      fieldCls: 'valores'
                  }]
              });
  </code></pre>
  
  <p>I think I have all the code like this example:
  <a href="http://dev.sencha.com/deploy/ext-3.4.0/examples/grid/grouping.html" rel="nofollow noreferrer">http://dev.sencha.com/deploy/ext-3.4.0/examples/grid/grouping.html</a></p>
  
  <p>I don't know why the grid doesn't group. I get all the data from a database with ajax and php that generate json. All fields shows up. The unique problem is the non grouping. I also get no errors.</p>
  
  <p>Anyone could help me with that?</p>