Add a Date header and a Message-ID header to the mail generated by mailer.py.

* tools/hook-scripts/mailer/mailer.py
  (MailedOutput.mail_headers): Add a Date and Message-Id header to the output.

Patch by: Nick Peelman <nick{_AT_}peelman.us>
(Tweaked by me to remove revision from Message-ID)


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1449592 13f79535-47bb-0310-9956-ffa450edef68