Bug 1837680: Add Message-ID header when missing at send

[mrenvoize]

Detect whether a Message-ID header has been included in the message passed to MessageToMTA and generate one if it's missing.

GMail and other modern mail services now expect a Message-ID as part of the message else they mark them as spam or drop them entirely.

** This is a port of the same bug fix submitted for Bugzilla 5.2 **