public function setReferrerPolicy($policy = self::SAME_ORIGIN)
    {
        $available = [
            self::NO_REFERRER,
            self::NO_REFERRER_WHEN_DOWNGRADE,
            self::ORIGIN,
            self::ORIGIN_WHEN_CROSS_ORIGIN,
            self::SAME_ORIGIN,
            self::STRICT_ORIGIN,
            self::STRICT_ORIGIN_WHEN_CROSS_ORIGIN,
            self::UNSAFE_URL,
        ];

        $this->checkValues($policy, $available);
        $this->headers['referrer-policy'] = $policy;

        return $this;
    }