[bug]DamgeModifier can't work with Skill's damage

Current behavior

DamgeModifier can't work with Skill's damage

For example, a skill:

  Skills:
  - damage{a=100;damagecause=FIRE} @eir{r=5}

A mob:

Dummy:
  Type: ZOMBIE
  Display: '&6Dummy'
  Health: 1000
  DamageModifiers:
  - FIRE 0.5
  - ENTITY_ATTACK 0.5
  Skills:
  - m{m="HP is <caster.hp>";delay=5} @PIR{r=50} ~onDamaged
  - message{m="Damaged by <skill.var.damage-type> for <skill.var.damage-amount>"} @PIR{r=50} ~onDamaged

then you summon the dummy and test cast the skill, you can see the message is

Damaged by UNDEFINED fo 50.0
HP is 900

so it show 50 damage,but truely damage 100.

if you change the damagecause to ENTITY_ATTACK,you will see the message is

Damaged by SKILL fo 100.0
HP is 900

the damage reduce can't work