public function addHeader(string $name, $value)
    {
        $lName = strtolower($name);
        if (isset($this->headers[$lName])) {
            $this->headers[$lName][1] = array_merge(
                $this->headers[$lName][1],
                (array) $value
            );
        } else {
            $this->headers[$lName] = [
                $name,
                (array) $value,
            ];
        }
    }