Trigger 'sync' event on prefill success

Trigger 'sync' event on prefill success.

Without doing so, a lot of the other Backbone behavior in many apps won't fire so there is little benefit from retrieving this cached object. 

Adding this ensures that a sync event is triggered each time the object is fetched regardless of where it came from (cache or server).

Also, make sure prefill success is called with the same arguments as success so such handlers can work the same.