public function addHeader($headerLine)
    {
        $parts = explode(':', $headerLine, 2);
        if (2 !== count($parts)) {
            throw new \InvalidArgumentException(
                sprintf('"%s" is not a valid HTTP header line', $headerLine)
            );
        }
        $name = trim($parts[0]);
        $value = trim($parts[1]);
        if ($this->response->hasHeader($name)) {
            $this->response = $this->response->withAddedHeader($name, $value);
        } else {
            $this->response = $this->response->withHeader($name, $value);
        }

        return $this;
    }