Core/Groups: Fixed UnitIsPartyLeader lua function.

https://github.com/TrinityCore/TrinityCore/commit/996e9115ac1104f23a52444c05c829ba1f1231ca
Core/Spells: Fixed AoE damage reduction auras, original patch by Unholychick.

Notes: Needs testing & Spell::DoAllEffectOnLaunchTarget needs updating.