public function user($username, Bag $bag = null)
    {
        $data = $this->createData(array(
            'api_token' => $this->apiToken,
            'username'  => strtoupper($username),
        ), $bag);

        $response = $this->send('yo/', InternalRequestInterface::METHOD_POST, $data);
        $content  = json_decode($response->getBody()->getContents());

        return (boolean) $content->success;
    }