AuthenticatedMessageHandler should skip if no provider has been set

This allows an application to use Flurl alongside OpenStack.NET without
us trying to authenticate at inappropriate times.

Author: carolynvs

src/corelib/Authentication/AuthenticatedMessageHandler.cs

@@ -21,6 +21,9 @@ public AuthenticatedMessageHandler(HttpMessageHandler innerHandler)
 
         protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
         {
+            if(AuthenticationProvider == null)
+                return await base.SendAsync(request, cancellationToken).ConfigureAwait(false);
+
             string token = await AuthenticationProvider.GetToken(cancellationToken).ConfigureAwait(false);
             request.Headers.SetAuthToken(token);