var response = await base.SendAsync(request, cancellationToken);
 
        if (credentials == null && response.StatusCode == HttpStatusCode.Unauthorized)
                Challenge(request, response);
 
 
        return response;

        return base.SendAsync(request, cancellationToken)
            .ContinueWith(task =>
            {
                var response = task.Result;
                if (credentials == null && response.StatusCode == HttpStatusCode.Unauthorized)
                    Challenge(request, response);
 
 
                return response;
            });