protected function prepareRequestHeaders(RequestInterface $request)
    {
        // @see http://en.wikipedia.org/wiki/User_agent
        $request->setHeader(
            'User-Agent',
            sprintf('%s/%s (PHP %s)', self::NAME, self::VERSION, phpversion())
        );
        $request->setHeader('X-BitPay-Plugin-Info', sprintf('%s/%s', self::NAME, self::VERSION));
        $request->setHeader('Content-Type', 'application/json');
        $request->setHeader('X-Accept-Version', '2.0.0');
    }