Allow multi-level STI

[meacuna]

If you have this scenario

class User
    acts_as_reader
end
class A
    acts_as_readable on: :created_at
end

class A < B
end

class B < C
end

And try to do something like

user = User.create!
c = C.create!

# Assume A.last is the same record as c
a = A.last
a.mark_as_read!(for: user)
user.have_read?(c)
# Expect true, but got false

I think it has to do with readable_parent behaviour.

I made a pull request to address this issue.