Failing AM test for legacy multipart alternative rendering

actionmailer/test/fixtures/test_mailer/multipart_alternative.html.erb

+<strong>foo</strong> <%= @foo %>
\ No newline at end of file

actionmailer/test/fixtures/test_mailer/multipart_alternative.plain.erb

+foo: <%= @foo %>
\ No newline at end of file

actionmailer/test/old_base/mail_render_test.rb

@@ -62,6 +62,33 @@ def initialize_defaults(method_name)
     super
     mailer_name "test_mailer"
   end
+
+  def multipart_alternative
+    recipients 'test@localhost'
+    subject    'multipart/alternative'
+    from       'tester@example.com'
+
+    build_multipart_message(:foo => "bar")
+  end
+
+  private
+    def build_multipart_message(assigns = {})
+      content_type "multipart/alternative"
+
+      part "text/plain" do |p|
+        p.body = build_body_part('plain', assigns, :layout => false)
+      end
+
+      part "text/html" do |p|
+        p.body = build_body_part('html', assigns)
+        p.transfer_encoding = "base64"
+      end
+    end
+
+    def build_body_part(content_type, assigns, options = {})
+      render "#{template}.#{content_type}", :body => assigns
+      # render options.merge(:file => "#{template}.#{content_type}", :body => assigns)
+    end
 end
 
 class FirstMailer < ActionMailer::Base
@@ -129,6 +156,16 @@ def test_no_instance_variable
     mail = RenderMailer.no_instance_variable.deliver
     assert_equal "Look, subject.nil? is true!", mail.body.to_s.strip
   end
+
+  def test_legacy_multipart_alternative
+    mail = RenderMailer.multipart_alternative.deliver
+    assert_equal(2, email.parts.size)
+    assert_equal("multipart/alternative", email.mime_type)
+    assert_equal("text/plain", email.parts[0].mime_type)
+    assert_equal("foo: bar", email.parts[0].body.encoded)
+    assert_equal("text/html", email.parts[1].mime_type)
+    assert_equal("<strong>foo</strong> bar", email.parts[1].body.encoded)
+  end
 end
 
 class FirstSecondHelperTest < Test::Unit::TestCase