only add hsts header with https.

fixes https://github.com/NginxProxyManager/nginx-proxy-manager/issues/1005
for more information look at: https://websistent.com/add-the-hsts-header-only-for-https-requests-nginx/

backend/templates/proxy_host.conf

@@ -1,6 +1,9 @@
 {% include "_header_comment.conf" %}
 
 {% if enabled %}
+
+{% include "_hsts_map.conf" %}
+
 server {
   set $forward_scheme {{ forward_scheme }};
   set $server         "{{ forward_host }}";