public function stats_send($template = null, $start_date = null, $end_date = null, array $data = [ ]) {
        $data['stat'] = 'send';

        if (!is_null($template)) {
            $data['template'] = $template;
        }
        if (!is_null($start_date)) {
            $data['start_date'] = $start_date;
        }
        if (!is_null($end_date)) {
            $data['end_date'] = $end_date;
        }

        return $this->stats($data);
    }