notifier_3.rb

class Notifier
  attr_reader :notifier

  def initialize(notifier)
    raise ArgumentError, 'Expected the notifier to respond to `notify`' unless notifier.respond_to?(:notify)
    @notifier = notifier
  end

  def notify(id, message)
    notifier.notify(id, prettify(message))
  end

  def prettify(message)
    HTMLCleaner.clean(message.strip)
  end
end

class Mailer < DefaultMailer
  def notify(email, message)
     mail(to: email) { render :plain, message }
  end
end

Notifier.new(Mailer.new).notify('foo@bar.com', 'hey there')
Notifier.new(SlackClient.new).notify('@kirill', 'hello, <strong>world</strong>')
```