check co_id before calling sponsorList

CoPeopleController contains in beforeFilter() this code:

// generate list of sponsors
$this->set('sponsors', $this->CoPerson->sponsorList($this->cur_co['Co']['id']));

This needs to be moved to a beforeRender() method because in beforeFilter() it is being called too early to prevent a REST call from being checked to see if there is a co_id that could be put into the sponsorList() call.