Apply irrelevant changes caused by touch

[wata727]

While doing some research, I noticed that models that include ActiveRecord::Bitemporal have unintended changes after being touched:

# bitemporal
employee.changes.keys # => []
employee.touch
employee.changes.keys # => ["valid_from", "transaction_from"]
employee.saved_changes.keys # => ["updated_at"]

# non-bitemporal
book.changes.keys # => []
book.touch
book,changes.keys # => []
book.saved_changes.keys # => ["updated_at"]

Perhaps adding changes unintentionally is not the expected behavior.

This PR changes the changes caused by touch to be moved saved_changes:

employee.changes.keys # => []
employee.touch
employee.changes.keys # => []
employee.saved_changes.keys # => ["valid_from", "transaction_from", "updated_at"]

As a side note, I noticed that unlike the non-bitemporal model, there was an issue where the dirty state before touch was unintentionally saved, but that's an issue that should be addressed in another PR.

[wata727]

This PR is frozen because #148 may fix this issue.