private function getByIndex($index)
    {
        if (!isset($this->headers[$index])) {
            return null;
        }
        if ($this->headerObjects[$index] === null) {
            $this->headerObjects[$index] = $this->headerFactory->newInstance(
                $this->headers[$index][0],
                $this->headers[$index][1]
            );
        }
        return $this->headerObjects[$index];
    }