Unit-0: Fixed bug with CounterActor (#352)

need to use the `FetchCounts.Subscriber` property, rather than `Context.Sender`

Author: Aaronontheweb

unit-0/completed/AkkaWordCounter/CounterActor.cs

@@ -44,10 +44,12 @@ protected override void OnReceive(object message){
                 _subscribers.Clear();
                 break;
             }
-            case FetchCounts when _doneCounting:
+            case FetchCounts fetchCounts when _doneCounting:
+            {
                 // instantly reply with the results
-                Sender.Tell(_tokenCounts.ToImmutableDictionary());
+                fetchCounts.Subscriber.Tell(_tokenCounts.ToImmutableDictionary());
                 break;
+            }
             case FetchCounts fetch:
             {
                 _subscribers.Add(fetch.Subscriber);