private static function createBaseClient($url, HandlerStack $handlerStack = null)
    {
        $httpClient = new \GuzzleHttp\Client(
            [
                'base_uri' => $url,
                'handler'  => $handlerStack,
                'headers'  => [
                    'Content-Type' => 'application/json',
                    'User-Agent'   => 'bunq-api-client:user',
                ],
            ]
        );

        return $httpClient;
    }