Authenticating handler

AuthenticatedHttpClientHandler.cs

    class AuthenticatedHttpClientHandler : HttpClientHandler
    {
        private readonly Func<Task<string>> getToken;

        public AuthenticatedHttpClientHandler(Func<Task<string>> getToken)
        {
            if (getToken == null) throw new ArgumentNullException("getToken");
            this.getToken = getToken;
        }

        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {
            // See if the request has an authorize header
            var auth = request.Headers.Authorization;
            if (auth != null)
            {
                var token = await getToken().ConfigureAwait(false);
                request.Headers.Authorization = new AuthenticationHeaderValue(auth.Scheme, token);
            }

            return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
        }
    }