title: How can I change/add params on a store
body: <p>On a sencha-touch, here's is my store declaration</p>
  
  <pre><code>Ext.regStore('newsStore',  {
      model: 'News',
      autoLoad: true,
      proxy: {
          type: 'ajax',
          url: '../news/list.form?format=json',
          reader: {
              type: 'json',
              root: ''
          }
      },                        
  });
  </code></pre>
  
  <p>How can I modify params? I tried</p>
  
  <pre><code>params:{ format: 'json'}
  </code></pre>
  
  <p>But its doesn't work!</p>