protected function makeHttpOptions($postString = '')
    {
        // Union custom Curl options and default ones.
        $options = array_replace(
            $this->options,
            $this->getDefaultHttpOptions()
        );

        if (!empty($postString)) {
            $options[CURLOPT_POST] = true;
            $options[CURLOPT_POSTFIELDS] = $postString;
        }

        return $options;
    }