MetaMTA - make test mail implementation support message id headers by default

Summary: Fixes T6372. Apparently ye olde error logs get some crazy spam action as is... Looking around at call sites, we do not specify $config (which could specify the supportage of message id header) so it seems correct to default this to something. I went with "true" as the spot we use this seems like pretty easy stuff that will always work.

Test Plan: lots of thinking

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6372

Differential Revision: https://secure.phabricator.com/D10749

src/applications/metamta/adapter/PhabricatorMailImplementationTestAdapter.php

@@ -75,7 +75,7 @@ final class PhabricatorMailImplementationTestAdapter
   }
 
   public function supportsMessageIDHeader() {
-    return $this->config['supportsMessageIDHeader'];
+    return idx($this->config, 'supportsMessageIDHeader', true);
   }
 
   public function send() {