code: func (k *Kite) handleHeartbeat(r *Request) (interface{}, error) {
  	req, err := newHeartbeatReq(r)
  	if err != nil {
  		return nil, err
  	}
  
  	k.heartbeatC <- req
  
  	return nil, req.ping()
  }
docs: // handleHeartbeat pings the callback with the given interval seconds.