public function refreshToken(AccessToken $accessToken)
    {
        // request data
        $refreshToken = $accessToken->getRefreshToken();
        $scope        = $accessToken->getScope();

        if (empty($refreshToken)) {
            throw new RuntimeException('No refresh token was set');
        }

        $data = array(
            'grant_type'    => 'refresh_token',
            'refresh_token' => $refreshToken,
        );

        if (!empty($scope)) {
            $data['scope'] = $scope;
        }

        // authentication
        $header = array();

        if ($this->type == self::AUTH_BASIC) {
            $header['Authorization'] = 'Basic ' . base64_encode($this->clientId . ':' . $this->clientSecret);
        }

        if ($this->type == self::AUTH_POST) {
            $data['client_id']     = $this->clientId;
            $data['client_secret'] = $this->clientSecret;
        }

        $request  = new PostRequest($this->url, $header, $data);
        $response = $this->httpClient->request($request);

        $data = Json\Parser::decode($response->getBody());

        if ($response->getStatusCode() == 200) {
            return $this->newToken($data);
        } else {
            throw new RuntimeException('Could not refresh access token');
        }
    }