This can be useful in after_commit callbacks such as:
after_save_commit :publish_status, if: :previously_activated?
def previously_activated?
status == 'activated' && (previously_new_record? || status_previously_changed?)
end
Previously, freezing a cloned ActiveRecord object froze the original
too. By cloning `@attributes` before freezing, we prevent cloned objects
(which in Ruby share state of ivars) from being effected by `#freeze`.
Resolves issue #4936, which has further information on this issue, as
well as steps to reproduce.
* Add a test case for `#freeze` not causing `cloned.frozen?` to be true.
* Clone @attributes before freezing in `ActiveRecord::Core`, then
reassign the cloned, frozen hash to the frozen model's `@attributes`
ivar.
/cc @steveklabnik
