title: Ext.data.store POST data in JSON format issue
body: <p>I'm trying to make EXT JSON store to send data using JSON, however it does not seem to be working.
  Here is simple code: </p>
  
  <pre><code>       var myStore = new Ext.data.Store({
      //model: 'User',
      proxy: {
          type: 'ajax',
          url: '/users.svc',
          reader: {
              type: 'json',
              root: 'users'
          },
          writer: {
              type: 'json',
              root: 'data'
          },
          actionMethods: {
              create: 'POST', read: 'POST', update: 'POST', destroy: 'POST'
          },
          extraParams: { test: 'test' }
      },
          listeners: {
              beforeload: function (store, operation, options) {
                  //alert(operation.params);
              }
          },
      autoLoad: true
  });
  </code></pre>
  
  <p>Since I defined JSON "writer", my expectation that parameterswould be send to server using JSON.
  However it's still doing regular POST with following body:
  <code>test=test&amp;page=1&amp;start=0&amp;limit=25</code></p>
  
  <p>While my expectation is that POST should have the following body: <code>{test:'test',page:1,start:0}</code></p>
  
  <p>I would appreciate any help</p>
  
  <p>P.S. I'm using EXTJS 4.0.7</p>