Issue #3317709 by ugas19: LegacyHelper emailToArray method doesn't save message changes

src/LegacyMailerHelper.php

@@ -60,7 +60,7 @@ class LegacyMailerHelper implements LegacyMailerHelperInterface {
   /**
    * {@inheritdoc}
    */
-  public function emailToArray(EmailInterface $email, array $message) {
+  public function emailToArray(EmailInterface $email, array &$message) {
     $message['subject'] = $email->getSubject();
     if ($email->getPhase() >= EmailInterface::PHASE_POST_RENDER) {
       $message['body'] = $email->getHtmlBody();

src/LegacyMailerHelperInterface.php

@@ -29,7 +29,7 @@ interface LegacyMailerHelperInterface {
    * @param array $message
    *   The array to fill.
    */
-  public function emailToArray(EmailInterface $email, array $message);
+  public function emailToArray(EmailInterface $email, array &$message);
 
   /**
    * Fills an Email from a message array.