issue: In-Reply-To Header

This addresses an issue raised in 5111 where the `in-reply-to` header is not
sent with outgoing emails. This is due to the `$recipients` variable being
an instance of `MailingList` which is not handled correctly in
`class.mailer.php`. This adds a new case to handle `MailingList` directly.

include/class.mailer.php

@@ -330,6 +330,9 @@ class Mailer {
 
                 $entry = null;
                 switch (true) {
+                case $recipients instanceof MailingList:
+                    $entry = $thread->getLastEmailMessage();
+                    break;
                 case $recipients instanceof TicketOwner:
                 case $recipients instanceof Collaborator:
                     $entry = $thread->getLastEmailMessage(array(