Handle properly the custom bukkit damage cause

[Lulu13022002]

The custom bukkit damage cause MELTING and POISON replace totally the vanilla ones resulting in incompatible vanilla behavior and a non future proof way. For example in vanilla when you throw a potion of poison on a silverfish near stone/infested brick it will destroy the nearby stones, but on paper it doesn't because of the custom poison replacing the magic one (so source == DamageSource.MAGIC become false). Also i know that the shallow clone that bukkit does isn't future proof but since the DamageCause will be deprecated it's not really a problem actually.