Fix broken unit tests.

Source/MQTTnet/Internal/Disposable.cs

@@ -2,32 +2,20 @@
 
 namespace MQTTnet.Internal
 {
-    public class Disposable : IDisposable
+    public abstract class Disposable : IDisposable
     {
-        protected bool IsDisposed => _isDisposed;
+        protected bool IsDisposed { get; private set; } = false;
 
         protected void ThrowIfDisposed()
         {
-            if (_isDisposed)
+            if (IsDisposed)
             {
                 throw new ObjectDisposedException(GetType().Name);
             }
         }
 
-
-        #region IDisposable Support
-       
-        private bool _isDisposed = false; // To detect redundant calls
-
         protected virtual void Dispose(bool disposing)
         {
-            if (disposing)
-            {
-                // TODO: dispose managed state (managed objects).
-            }
-
-            // TODO: free unmanaged resources (unmanaged objects) and override a finalizer below.
-            // TODO: set large fields to null.
         }
 
         // TODO: override a finalizer only if Dispose(bool disposing) above has code to free unmanaged resources.
@@ -40,18 +28,17 @@ namespace MQTTnet.Internal
         // This code added to correctly implement the disposable pattern.
         public void Dispose()
         {
-            if (_isDisposed)
+            // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
+
+            if (IsDisposed)
             {
                 return;
             }
 
-            _isDisposed = true;
+            IsDisposed = true;
 
-            // Do not change this code. Put cleanup code in Dispose(bool disposing) above.
             Dispose(true);
-            // TODO: uncomment the following line if the finalizer is overridden above.
-            // GC.SuppressFinalize(this);
+            GC.SuppressFinalize(this);
         }
-        #endregion
     }
 }