public function setHeader($header)
    {
        $headers = [];

        if (is_array($header)) {
            foreach ($header as $key => $value) {
                $headers[] = $key . ': ' . $value;
            }
        } else {
            $headers[] = $header;
        }

        $this->setOpt(CURLOPT_HTTPHEADER, $headers);

        return $this;
    }