Fix Mailgun Reply-To handling

Summary:
When sending the "Reply-To" header to Mailgun, Phabricator would
previously send two headers for every "Reply-To": "Reply-To[0][email]" and
"Reply-To[0][name]". Instead, explicitly build the header as specified by RFC
2822 and send it to Mailgun pre-baked.

Pretty sure this bug was a cargo-cult from the Sendgrid code, where (apparently)
this actually works.

Test Plan:
Triggered an email from Phabricator, saw that the header was sent
properly.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8645

Author: zenazn

src/applications/metamta/adapter/PhabricatorMailImplementationMailgunAdapter.php

@@ -18,10 +18,7 @@ public function addReplyTo($email, $name = '') {
     if (empty($this->params['reply-to'])) {
       $this->params['reply-to'] = array();
     }
-    $this->params['reply-to'][] = array(
-      'email' => $email,
-      'name'  => $name,
-    );
+    $this->params['reply-to'][] = "{$name} <{$email}>";
     return $this;
   }
 
@@ -92,7 +89,7 @@ public function send() {
 
     if (idx($this->params, 'reply-to')) {
       $replyto = $this->params['reply-to'];
-      $params['h:reply-to'] = $replyto;
+      $params['h:reply-to'] = implode(', ', $replyto);
     }
 
     if (idx($this->params, 'ccs')) {