Fix a bug in ActiveRecord::Store accessors

There is a bug on tracking changes on attributes created by `ActiveRecord::Store`. This would happen before:

```ruby
class User < ActiveRecord::Base
  store :properties, accessors: [:age]
end

user = User.create!(age: 21)
user.age = 22
user.changes # => {"properties" => [{:age => 22}, {:age => 22}]}
```