Adjusted compatibility with Rails version >= 3.2.9

Refers the issue #54. 
The change in Rails that caused the mismatch is seen here:
* https://github.com/rails/rails/blob/master/activerecord/lib/active_record/attribute_methods/dirty.rb#L87
* http://weblog.rubyonrails.org/2012/10/29/ann-rails-3-2-9-rc1-has-been-released/