Hook the real global _G.UnitHealth for feign death

Inside a RegisterModule body, `function UnitHealth(...)` defines UnitHealth on
the pfUI environment, not the real global -- so the feign-death real-HP fix
only reached callers that resolve UnitHealth through pfUI's env, and missed
_G consumers (Blizzard frames, other addons). Hook _G.UnitHealth explicitly
(and capture oldUnitHealth from _G) so the un-gate applies everywhere.

modules/feigndeath.lua

@@ -1,6 +1,6 @@
 pfUI:RegisterModule("feigndeath", function ()
-  local oldUnitHealth = UnitHealth
-  function UnitHealth(unit)
+  local oldUnitHealth = _G.UnitHealth
+  _G.UnitHealth = function(unit)
     if UnitIsFeignDeath(unit) then
       local hp = GetUnitField(unit, "health")
       if hp and hp > 0 then return hp end