mailer: Try harder to find References header

Ensure that the References header that is included with outbound email
definitely includes the References header from an email received into the
system, if at all possible.

include/class.mailer.php

@@ -304,11 +304,13 @@ class Mailer {
         if (isset($options['thread'])
             && $options['thread'] instanceof ThreadEntry
         ) {
-            $headers += array('References' => $options['thread']->getEmailReferences());
             if ($irt = $options['thread']->getEmailMessageId()) {
                 // This is an response from an email, like and autoresponse.
                 // Web posts will not have a email message-id
-                $headers += array('In-Reply-To' => $irt);
+                $headers += array(
+                    'In-Reply-To' => $irt,
+                    'References' => $options['thread']->getEmailReferences()
+                );
             }
             elseif ($parent = $options['thread']->getParent()) {
                 // Use the parent item as the email information source. This