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

src/applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php

@@ -68,7 +68,7 @@ final class PhabricatorMailImplementationMailgunAdapter
   }
 
   public function supportsMessageIDHeader() {
-    return false;
+    return true;
   }
 
   public function send() {