Fix auto-leave-on-end: true error.

The ConcurrentModificationException happens because the player leave
procedure alters the `specPlayers` set, and this is the set we iterate
during the automatic spectator kicking procedure. By making a copy of
the set and iterating the copy, we circumvent this problem. Java 101,
really, but when the code is stateful and complex, it's "fair enough"
that it slipped through the cracks. It's also impossible to reproduce
without at least two players, so go figure...

Fixes #802

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

@@ -665,7 +665,8 @@ public class ArenaImpl implements Arena
 
         // Auto-leave
         if (settings.getBoolean("auto-leave-on-end", false)) {
-            specPlayers.forEach(this::playerLeave);
+            List<Player> spectators = new ArrayList<>(specPlayers);
+            spectators.forEach(this::playerLeave);
         }
 
         return true;