title: Update value with Sencha Touch / Ext JS localstorage duplicated entries
body: <p>i want to update a value of an record in my localstorage. At the moment it will be added a new record with the same id, duplicated. For example if the function update will be executed the row that contains all the ids (here 'test-proxy') then it will be appended the same id again.</p>
  
  <pre><code>model.Test = Ext.regModel('model.Test', {
      idProperty: 'id',
  fields : [ {
      name : 'id',
      type : 'integer'
  }, {
      name : 'name',
      type : 'string'
  }
  ]});
  
  
  //Store
  store = new Ext.data.Store({
  model : 'model.Test',
  sorters : 'title',
  storeId : 'test',
   proxy: {
      type: 'localstorage',
      id  : 'test-proxy',
      idProperty: 'id'
  }
  });
  
  updateName = function(id) {
      store.read();
      var item = store.getAt(id-1).data;
  
      //toggle
      item.name = "Update"
      store.add(item);
      store.sync();
      console.log(store.getAt(id-1));
  }
  </code></pre>