Core/Creatures: Implemented unkillable flag (prevent health from going below 1)

Port From (https://github.com/TrinityCore/TrinityCore/commit/7ea438ebdca93874776e02e9985db1da0a53f2fb)

Source/Game/Entities/Creature/Creature.cs

@@ -3187,6 +3187,14 @@ namespace Game.Entities
         public override void SetImmuneToPC(bool apply) { SetImmuneToPC(apply, HasReactState(ReactStates.Passive)); }
         public override void SetImmuneToNPC(bool apply) { SetImmuneToNPC(apply, HasReactState(ReactStates.Passive)); }
 
+        void SetUnkillable(bool unkillable)
+        {
+            if (unkillable)
+                _staticFlags |= CreatureStaticFlags.Unkillable;
+            else
+                _staticFlags &= ~CreatureStaticFlags.Unkillable;
+        }
+
         public bool IsInEvadeMode() { return HasUnitState(UnitState.Evade); }
         public bool IsEvadingAttacks() { return IsInEvadeMode() || CanNotReachTarget(); }
 

Source/Game/Entities/Unit/Unit.cs

@@ -2591,6 +2591,10 @@ namespace Game.Entities
 
                 duel_hasEnded = true;
             }
+            else if (victim.IsCreature() && damageTaken >= health && victim.ToCreature().HasFlag(CreatureStaticFlags.Unkillable))
+            {
+                damageTaken = health - 1;
+            }
             else if (victim.IsVehicle() && damageTaken >= (health - 1) && victim.GetCharmer() != null && victim.GetCharmer().IsTypeId(TypeId.Player))
             {
                 Player victimRider = victim.GetCharmer().ToPlayer();