Core/Pet: load template immunities when creating (except for hunters)

Port From (https://github.com/TrinityCore/TrinityCore/commit/053d0cfda5a96160f9e480e29bd432bc59568f86)

Source/Game/Entities/Creature/Creature.cs

@@ -1698,7 +1698,7 @@ namespace Game.Entities
                 ForcedDespawn(msTimeToDespawn, forceRespawnTimer);
         }
 
-        void LoadMechanicTemplateImmunity()
+        public void LoadMechanicTemplateImmunity()
         {
             // uint32 max used for "spell id", the immunity system will not perform SpellInfo checks against invalid spells
             // used so we know which immunities were loaded from template

Source/Game/Entities/Pet.cs

@@ -356,6 +356,8 @@ namespace Game.Entities
             if (owner.IsTypeId(TypeId.Player) && IsControlled() && !IsTemporarySummoned() && (GetPetType() == PetType.Summon || GetPetType() == PetType.Hunter))
                 owner.ToPlayer().SetLastPetNumber(petId);
 
+            // must be after SetMinion (owner guid check)
+            LoadMechanicTemplateImmunity();
             m_loading = false;
 
             return true;