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