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/Explosion.java

@@ -144,6 +144,7 @@ public class Explosion {
                     Bukkit.getPluginManager().callEvent(event);
 
                     if (!event.isCancelled()) {
+                        damagee.setLastDamageCause(event);
                         entity.damageEntity(DamageSource.EXPLOSION, event.getDamage());
                         entity.motX += d0 * d10;
                         entity.motY += d1 * d10;