public function issue()
    {
        // If there is no next URL, just output the headers as expected.
        foreach ($this->headers as $key => $value) {
            header("{$key}: {$value}", true);
        }

        // Output content type.
        if ($this->contentType) {
            header("content-type: {$this->contentType}; charset={$this->encoding}");
        }

        // Output status code.
        $statusMessage = sprintf('%d %s',
            $this->status,
            HttpClient::getStatusCodeMessage($this->status)
        );
        header('Status: ' . $statusMessage);
    }