private function getHeader($key)
    {
        if (! $this->headers) {
            $this->headers = $this->getHeaders();
        }
        if (array_key_exists($key, $this->headers)) {
            //if redirects encountered, return the final values
            if (is_array($this->headers[$key])) {
                return end($this->headers[$key]);
            } else {
                return $this->headers[$key];
            }
        }
    }