Dup the changed_attributes otherwise we could lose them

4-0-stable is falling after https://github.com/rails/rails/commit/8de5d7622df68dd27c12b115da71fc9e94e490e9 because `changed_attributes` is being mutated on the `save` method, and as `@_start_transaction_state` only have a reference to that object, it lose those changes.

master and 4.1 are not falling because commit  https://github.com/rails/rails/commit/9aa1a3d85327fa0a3055b5b757a0be092ce582f7 removes the mutation on `changed_attributes`.

To make sure we keep those changes, we need to `.dup` the `changed_attributes` to hold a new instance of that Hash.

review @rafaelfranca @carlosantoniodasilva @chancancode