Make sure to shutdown receive socket when client has closed.

Author: drieseng

src/Renci.SshNet.Tests/Common/AsyncSocketListener.cs

@@ -127,6 +127,8 @@ private void ReadCallback(IAsyncResult ar)
             else
             {
                 SignalDisconnected(handler);
+                handler.Shutdown(SocketShutdown.Both);
+                handler.Close();
             }
         }