public function send(Message $message)
    {
        return $this->guzzleClient->post(
            $this->getApiUrl(),
            [
                'headers' => [
                    'Authorization' => sprintf('key=%s', $this->apiKey),
                    'Content-Type' => 'application/json'
                ],
                'body' => json_encode($message)
            ]
        );
    }