Rails - Jul 2009 - 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-hcDgGtZH8xNBDgjK7y7TUQ@public.gmane.org''
    @from       =
''test.sender-hcDgGtZH8xNBDgjK7y7TUQ@public.gmane.org''
    @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:

--------------------------------------------------------------------
Date: Thu, 30 Jul 2009 17:45:04 -0400
From: test.sender-hcDgGtZH8xNBDgjK7y7TUQ@public.gmane.org>
To: test.recipient-hcDgGtZH8xNBDgjK7y7TUQ@public.gmane.org
Subject: Thank you for registering
Mime-Version: 1.0
Content-Type: multipart/alternative;
boundary=mimepart_4a72146057a58_11b414263a41e


--mimepart_4a72146057a58_11b414263a41e
Mime-Version: 1.0
Content-Type: text/plain; charset=iso-2022-jp
Content-Transfer-Encoding: 7bit
Content-Disposition: inline


Message text from confirmation.text.plain.erb

--mimepart_4a72146057a58_11b414263a41e
Mime-Version: 1.0
Content-Type: text/plain; charset=iso-2022-jp
Content-Transfer-Encoding: 7bit
Content-Disposition: inline


Message text from confirmation.text.html.erb

--mimepart_4a72146057a58_11b414263a41e--
--------------------------------------------------------------------

The second message part should have a content-type of text/html, but
it doesn''t (any more; this worked in the past).

This happens regardless of whether I run test\unit
\notifications_test.rb directly or whether I run it via rake
test:units.

Searching for more information led me to a similar problem with Rails
2.3.2 and I tried the fix suggested at
https://rails.lighthouseapp.com/projects/8994/tickets/2263-rails-232-breaks-implicit-multipart-actionmailer-tests#ticket-2263-23
but it isn''t helping.

Does anyone know why the implicit multipart functionality isn''t
working for me?

Thanks,

Sven