Fix change of vritual attribute can not be tracked.

[bealking]

Some gems like "acts-as-taggable-on" may add some vritual attribute/method that inherits activerecord dirty feature. Please see the follow links:

https://github.com/mbleigh/acts-as-taggable-on/blob/64ed839a8a6021c719fbff9c6bdf74a4fcd65f18/lib/acts_as_taggable_on/taggable/core.rb#L193 https://github.com/mbleigh/acts-as-taggable-on/blob/8e64c3d4a81cfbb8af621228eae36a65c1f94501/lib/acts_as_taggable_on/taggable/dirty.rb#L14

audited_changes uses rails "[]" method to fetch the new value of changed attribute which leads those gems that didn't extend this method to return nil (please see the test example added).

so I change the method to public_send to solve this problem.