Restore timestamps in model rollback

The current implementation of rollback_active_record_state! does some housekeeping like restoring @new_record. If you rollback a model after it was once saved like this

dev.rollback_active_record_state! do
  Developer.transaction do
    dev.save
    # other stuff that may raise an exception
  end
end

you may end up with a new_record? that has an initialized created_at for example.

This patch fixes that for timestamps.