protected function buildHeaders($requestHeaders)
    {
        $requestHeaders = $requestHeaders + $this->requestHeaders;
        $resultHeaders = [];

        foreach ($requestHeaders as $headerName => $headerValue) {

            if (is_array($headerValue)) {
                $headerValue = implode(',', $headerValue);
            }

            $resultHeaders[] = ucfirst($headerName) . ': ' . $headerValue;
        }

        return $resultHeaders;
    }