fixed managed client so it does not send disconnect packet when disposed

Source/MQTTnet/Internal/Disposable.cs

@@ -44,12 +44,13 @@ namespace MQTTnet.Internal
             {
                 return;
             }
+
+            _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);
-
-            _isDisposed = true;
         }
         #endregion
     }