I added: proxy_set_header X-Forwarded-Ssl on;

I think if nginx is used as main back-end (without Apache), this header means nothing. In this case just refer this guide: https://scotthelme.co.uk/setting-up-hsts-in-nginx/

If I'm not right, and there is also Apache server, follow: http://stackoverflow.com/questions/16042647/whats-the-de-facto-standard-for-...

С наилучшими пожеланиями, Яницкий Вадим.