public function add($path, $position = 0)
    {
        if (!\is_string($path)) {
            throw new \InvalidArgumentException('$path must be only string type');
        }

        if (!$position) {
            $this->headers[$path] = $path;
        } else {
            switch ($position) {
                case self::POSITION_FIRST:
                    $this->headersFirst[$path] = $path;
                    break;
                case self::POSITION_LAST:
                    $this->headersLast[$path] = $path;
                    break;
                default:
                    break;
            }
        }
    }