title: After updating a record via store.sync()  in Ext JS 4, the record is doubled
body: <p>My json-store contains one record. This record is dirty after changing a field and correctly after store.sync() a POST is sended with action=update and the record data in the body.</p>
  
  <p>On the server side I commit the update and send back the record (with success:true) as it was posted originally (all fields are included).</p>
  
  <p>Afterwards the store has doubled the record, which means the store has now two (2) identical records (same id etc.).</p>
  
  <p>I played around with all options I could imagine of (root [on/off], idProperty (set/unset) etc.) the effect is always the same.</p>
  
  <p>Any hint what I am doing wrong?</p>