Make Server.getOnlinePlayers returns immutable collection, like it said

in docs. This is also fix crash with /stop and /restart commands while
any players had been online.

src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -452,7 +452,7 @@ public final class CraftServer implements Server {
 
     @Override
     public List<CraftPlayer> getOnlinePlayers() {
-        return this.playerView;
+        return ImmutableList.copyOf(this.playerView);
     }
 
     @Override