Fix check for arena monsters in monster damage logic.

This commit changes how the final "catch all" works for incoming monster damage. Instead of checking if the damager entity is a LivingEntity, we check if it is a monster of the arena and then proceed as normal.

Note that this change may make it possible for monsters in the arena to take damage from non-arena entities. The so-called "catch all" doesn't really catch everything, but it does catch the infighting that it was designed to catch.

One "not thought of" damage source is that of player-made golems, so coincidentally, this commit fixes #550.

src/main/java/com/garbagemule/MobArena/ArenaListener.java

@@ -763,8 +763,7 @@ public class ArenaListener
             ArenaPlayerStatistics aps = arena.getArenaPlayer(p).getStats();
             aps.add("dmgDone", event.getDamage());
         }
-        //TODO add in check for player made golems doing damage
-        else if (damager instanceof LivingEntity) {
+        else if (monsters.getMonsters().contains(damager)) {
             if (!monsterInfight)
                 event.setCancelled(true);
         }