Admit to responding to original mail methods

Rounds out the duck typing, allowing consumers to ask the decoy if it will respond before attempting a particular method call.

This allows, e.g., `Rails::MailersController` to [ask the decoy if it has a `decoded` method][1] that it can use to display a preview of the mail message.

[1]: https://github.com/rails/rails/blob/6c5f43bab8206747a8591435b2aa0ff7051ad3de/railties/lib/rails/mailers_controller.rb#L29