Issue#3021619: Mail HTML to text prevent failure.

Closes #3021619

core/lib/Drupal/Core/Mail/MailFormatHelper.php

@@ -192,11 +192,11 @@ public static function htmlToText($string, $allowed_tags = NULL) {
             break;
 
           case 'h3':
-            $indent[] = '.... ';
+            $indent[] = '---- ';
             break;
 
           case 'h4':
-            $indent[] = '.. ';
+            $indent[] = '-- ';
             break;
 
           case '/blockquote':