devel.mail.inc doesn't provide a label for the Subject header in emails written to tmp/devel-mails

I ran into this while testing #1560012: Port per-user issue notification email functionality to D7. It's minor, but annoying. If you use devel.mail.inc, you see stuff like this:

From: "admin (D7 Project)" <admin@example.com>
Date: Thu, 23 May 2013 15:23:17 -0400
...
To: admin@example.com
[Task] Yet another new title
Issue status update for:
http://localhost/d7project/node/30
Update issue:
http://localhost/d7project/node/30/edit

It should really look like this:

From: "admin (D7 Project)" <admin@example.com>
Date: Thu, 23 May 2013 15:23:17 -0400
...
To: admin@example.com
Subject: [Task] Yet another new title

Issue status update for:
http://localhost/d7project/node/30
Update issue:
http://localhost/d7project/node/30/edit

At first, I thought my email generation code was broken, but upon closer inspection, it was just how devel.mail.inc is writing this stuff to disk. :/