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

@@ -132,7 +132,7 @@ public IBasicConsumer DefaultConsumer
             set => InnerChannel.DefaultConsumer = value;
         }
 
-        public bool IsClosed => _innerChannel != null && _innerChannel.IsClosed;
+        public bool IsClosed => !IsOpen;
 
         public bool IsOpen => _innerChannel != null && _innerChannel.IsOpen;