Problem with implicit multipart emails using ActionMailer in Rails 2.3.3

With earlier versions of Rails ActionMailer used to implicitly send
multipart (text/plain and text/html) email messages for me, but it
looks like now it's sending two text/plain parts.

Simplified details:

Here's my model:

class Notifications < ActionMailer::Base
  def confirmation(sent_at = Time.now)
    @subject = 'Thank you for registering'
    @body = {}
    @recipients = 'test.recipient@example.com'
    @from = 'test.sender@example.com'
    @sent_on = sent_at
    @headers = {}
  end
end

I have two views called confirmation.text.plain.erb and
confirmation.text.html.erb

In a unit test, for debug purposes, I have:
  sent = Notifications.create_confirmation
  puts sent.to_s

The resulting message looks like this: