public function refreshToken($refreshToken)
  {
    $this->refreshTokenRequest(
        array(
          'client_id' => $this->client->getClassConfig($this, 'client_id'),
          'client_secret' => $this->client->getClassConfig($this, 'client_secret'),
          'refresh_token' => $refreshToken,
          'grant_type' => 'refresh_token'
        )
    );
  }