Return updated item on DataStore.put()

Introducing a new variable `item` to store returned object from store (JsonRest). Then having the promise resolve to the new `item` instead of original `object`. Best explained with an example: If we pass `{name: 'dojo'}` to `store.newItem()`, then we will get something like `{id: 99, name: 'dojo', 'date_created': ...}` returned from well-built REST server. Now that we have a deferred/promise here (since last commit), we would expect the resolved value to be the new value returned from server, not our original input.