Add serialization of SpectatorMessage

Projekte/mdga/client/src/main/java/pp/mdga/client/server/MdgaServer.java

@@ -139,6 +139,7 @@ private void initializeSerializables() {
         Serializer.registerClass(UpdateTSKMessage.class);
         Serializer.registerClass(WaitPieceMessage.class);
         Serializer.registerClass(IncorrectRequestMessage.class);
+        Serializer.registerClass(SpectatorMessage.class);
         Serializer.registerClass(Player.class);
         Serializer.registerClass(Statistic.class);
         Serializer.registerClass(Board.class);