AutorecoveringModel.IsClosed should return true if model is unusable

As per the description of [`IsClosed` on `IModel.cs`](https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/4d624be819061c9272c406a7c5250ee24eaa759f/projects/RabbitMQ.Client/client/api/IModel.cs#L85-L88), this property should "return[s] true if the model is no longer in a state where it can be used". At present this returns `false` if the `_delegate` has been disposed of - which I would say indicates that the model "is no longer in a state where it can be used".

Fixes regression introduced as part of 54f7840 by aligning the behaviour of `IsClosed` with the underlying [implementation](https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/c9201e4f4ffb62206ca9c7bbca76ea68fe24345f/projects/RabbitMQ.Client/client/impl/ModelBase.cs#L161-L164) that would have been called were the delegate not disposed of.

Author: onematchfox

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;
             }
         }