add unit method SetImmuneTo

[55Honey]

Adds a new method to add and remove mechanic immunities.

Tested with:

local function ImmunityOff(eventid, delay, repeats, creature)
    creature:SetImmuneTo(26, false)
    creature:SendUnitSay("I am interruptable now.", 0)
end

local function Cast(eventid, delay, repeats, creature)
    creature:CastSpell(creature, 57775, false)
end

local function EnterCombat(event, creature, target)
    creature:SetImmuneTo(26, true)
    creature:SendUnitSay("You can not interrupt me.", 0)

    creature:RegisterEvent(Cast, 5500, 0)
    creature:RegisterEvent(ImmunityOff, 15000, 1)
end

local function LeaveCombat(event, creature)
    creature:RemoveEvents()
end

RegisterCreatureEvent(113, 1, EnterCombat)
RegisterCreatureEvent(113, 2, LeaveCombat) -- OnLeaveCombat
RegisterCreatureEvent(113, 4, LeaveCombat) -- OnDied