Override role as TANK based on MT assignments

This is particularly helpful in fights like High King Maulgar,
Illidari Council or Kael'Thas where a DPS player is temporarily
made into a tank.

src/types/unitgroup.lua

@@ -53,8 +53,8 @@ local function UnitFrameDefaultCompare(lhs, rhs)
         end
 
         -- tank < healer < damage < ?
-        local lrole = RoleValue(UnitGroupRolesAssigned(lunit))
-        local rrole = RoleValue(UnitGroupRolesAssigned(runit))
+        local lrole = RoleValue(lhs:GetRole())
+        local rrole = RoleValue(rhs:GetRole())
 
         if lrole ~= rrole then
             return lrole < rrole