title: ComboBox in editable grid: can't see the value
body: <p>I have an editable grid that uses a store. I want to insert a combobox in one of the fields.
  This is my store for the grid:</p>
  
  <pre><code>new Ext.data.Store({  ....
   proxy: new Ext.data.HttpProxy......
   reader: new Ext.data.JsonReader({   
              root: 'rows',
              fields: [..... {name:'wid', mapping: 'wid'},
  </code></pre>
  
  <p>There is another store for combobox only, which has 'wid' and 'name' fields.
  In my column model:</p>
  
  <pre><code> header: 'Worker',
      dataIndex: 'wid',
      editor: new Ext.grid.GridEditor(workerCmb),
      renderer:function(value, p, record){
      return record.data['name'];}
  </code></pre>
  
  <p>And the combo itself:</p>
  
  <pre><code>  valueField: 'wid',
              displayField: 'name',
  </code></pre>
  
  <p>When the grid is loaded its field "Worker" is empty (it is ok), but there is no combobox in it. When I start editing it, I see all the list. After editing the 'id' is saved to the store, but the 'name' is not shown, neither is the combobox.
  What am I doing wrong?</p>