Allow mails sent via MailHandler to specify a reply-to address

When using mailHandler to send email adding Reply-To in $params['headers'] doesn't work. It seams that we have to set reply-to parameter in mailManager.

Not sure if this is right approach but it works for my use case when sending email from EventSubscriberInterface with:

$params = [
   'id' => 'order_receipt',
   'from' => $from,
   'order' => $order,
   'reply-to' => $order->getStore()->getEmail(), 
];

As a side note, I think that it could be safer to send order receipt emails from \Drupal::config('system.site')->get('mail') with reply-to $order->getStore()->getEmail() or even better to have From address and Reply-To address in Store settings.

The problem with current setup is that order receipt will fail when using Office365 as SMTP server and probably some other providers that require same From address and account username.