[WebSocket] Fix peer disconnection not closing the network layer.

We were removing it from the MultiplayerAPI but not immediately
destroying it (to avoid tricky in-signal de-referencing).
We still need to properly close the connection in this case if we want
the remote peer to be notified immediately.

networking/websocket_multiplayer/script/main.gd

@@ -79,6 +79,10 @@ func _on_Host_pressed():
 
 
 func _on_Disconnect_pressed():
+	if peer is WebSocketServer:
+		peer.stop()
+	elif peer is WebSocketClient:
+		peer.disconnect_from_host()
 	_close_network()