Fixes so that conn.unsubscribe does not return null (#347)

Extracts CompletedTask and uses a completed task instead of null.

src/NATS.Client/Conn.cs

@@ -96,10 +96,6 @@ internal enum ClientProtcolVersion
     // interfaces.
     public class Connection : IConnection, IDisposable
     {
-#if NET45
-        private static readonly Task CompletedTask = Task.FromResult(true);
-#endif
-
         Statistics stats = new Statistics();
 
         // NOTE: We aren't using Mutex here to support enterprises using
@@ -3453,23 +3449,15 @@ public IAsyncSubscription SubscribeAsync(string subject, string queue, EventHand
         // Use Subscription.Unsubscribe()
         internal Task unsubscribe(Subscription sub, int max, bool drain, int timeout)
         {
-#if NET45
-            var task = CompletedTask;
-#else
-            var task = Task.CompletedTask;
-#endif
+            var task = CompletedTask.Get();
+
             lock (mu)
             {
                 if (isClosed())
                     throw new NATSConnectionClosedException();
 
-                Subscription s;
-                if (!subs.TryGetValue(sub.sid, out s)
-                    || s == null)
-                {
-                    // already unsubscribed
-                    return null;
-                }
+                if (!subs.TryGetValue(sub.sid, out var s) || s == null)
+                    return task;
 
                 if (max > 0)
                 {

src/NATS.Client/Internals/CompletedTask.cs

@@ -0,0 +1,30 @@
+// Copyright 2015-2018 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using System.Threading.Tasks;
+
+namespace NATS.Client.Internals
+{
+    internal static class CompletedTask
+    {
+#if NET45
+        private static readonly Task Completed = Task.FromResult(true);
+#endif
+        internal static Task Get() =>
+#if NET45
+            Completed;
+#else
+            Task.CompletedTask;
+#endif
+    }
+}