Made upsert return reference to created document

When upsert is true and it can't find an existing record to match the query, it calls self._insert which will return a ref to the newly created doc as the 2nd param to the callback, but the upsert logic wasn't catching a 2nd param (only the first param: err) so I just added the 2nd param as 'newDoc' and passed it to the upsert callback instead of 'true'.