Merge pull request #1174 from onematchfox/fix/autorecoveringmodel-isclosed

`AutorecoveringModel.IsClosed` should return `true` if model is unusable

Author: michaelklishin

projects/RabbitMQ.Client/client/impl/AutorecoveringModel.cs

@@ -360,14 +360,7 @@ public bool IsClosed
         {
             get
             {
-                if (_delegate == null)
-                {
-                    return false;
-                }
-                else
-                {
-                    return _delegate.IsClosed;
-                }
+                return !IsOpen;
             }
         }