Apply setLastDamageCause after processing events and the event has not been canceled. Fixes BUKKIT-1881

This allows previous causes to be available during the event, as well as making the damage cause a valid one. If EntityDamageEvent is canceled, then it's not the last DamageCause.

Also prevents setting DamageCause involuntarily through construction.

src/main/java/net/minecraft/server/BlockCactus.java

@@ -94,6 +94,7 @@ public class BlockCactus extends Block {
             world.getServer().getPluginManager().callEvent(event);
 
             if (!event.isCancelled()) {
+                damagee.setLastDamageCause(event);
                 entity.damageEntity(DamageSource.CACTUS, event.getDamage());
             }
             return;