mailgun support messageids. supportsMessageIDHeader accidentally returned false.

Summary: The adapter was mostly copy-paste, and I missed the supportsMessageIDHeader stuff.

Test Plan: Sent a message, checked headers.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9715