GetResponse method
Implementation
@override
Future<IEwsHttpWebResponse> GetResponse() async {
  final request = await _InternalGetRequest();
  final HttpClientResponse response = await request.close();
  _httpClient.close();
  if (response.statusCode < 200 || response.statusCode >= 300) {
    throw new WebException(
        WebExceptionStatus.ProtocolError, _request, response);
  }
  return EwsHttpWebResponse(this, response);
}