I duplicated your test but got the expected result with no extra blank line.

I'm using Ubuntu 8.04 Hardy. For this test I used the following newer version modules; all others are the Hardy default. I'm also using exim4 for the MTA instead of postfix, but I don't think that should matter.

Email-Date-Format-1.002
Email-MIME-1.902
Email-MIME-Encodings-1.313
Email-Simple-2.100
Email-Stuff-2.101