Issue #3388584: Wrong case for reply-to header after using MailManagerInterface::mail()

Closes #3388584

src/LegacyMailerHelper.php

@@ -93,7 +93,7 @@ class LegacyMailerHelper implements LegacyMailerHelperInterface {
       }
 
       // Copy the header.
-      $message['headers'][$name] = $header->getBodyAsString();
+      $message['headers'][$lc_name] = $header->getBodyAsString();
       if (!empty(self::ADDRESS_HEADERS[$lc_name])) {
         // Also copy directly to $message.
         $message[$lc_name] = $message['headers'][$name];