Fix CanCan::RulesCompressor for STI subclasses

[takayamaki]

This pull request fixes in the RulesCompressor class related to STI subclasses.

This PR may fix #874.

expexted and actual behavior

class Vehicle < ApplicationRecord; end
class Car < Vehicle; end
class Motorbike < Vehicle; end

Car.create!
Motorbike.create!

ability = Ability.new()
car = ability.can :read, Car
motorbike = ability.can :read, Motorbike

Vehicle.accessible_by(ability)
# expected => [car, motorbike]
# actual => [motorbike]
# missing => [car]

[23tux]

Whats the state of this PR? We still have the issue from #874

[23tux]

Alas, no response so far. It would be great if anyone from the maintainer could accept this PR, this would really help us. It seems that the whole repository is dead, last commit to the develop branch was January 2025 :(