jlxw

dirty methods do not work when tag_list changed using 'add' and 'remove' methods

10

``` Loading test environment (Rails 4.0.0) irb(main):001:0> i = Post.new => # irb(main):002:0> i.tag_list => [] irb(main):003:0> i.tag_list_changed? => false irb(main):004:0> i.tag_list.add('sfw') => ["sfw"] irb(main):005:0> i.tag_list_changed? => false irb(main):006:0> i.tag_list...