Fix Airsonic subdomain webUI http request on 443

Issue:  
When streaming via the webui on https://airsonic.myaddress.com opening a stream generates a url that looks like http://airsonic.myaddress.com:443/stream?player=5&id=176
Which nginx sees as a bad request as it's a http request on port 443.  

Adding `proxy_set_header X-Forwarded-Host  $http_host;` fixes this issue.

airsonic.subdomain.conf.sample

@@ -25,6 +25,7 @@ server {
         include /config/nginx/proxy.conf;
         resolver 127.0.0.11 valid=30s;
         set $upstream_airsonic airsonic;
+        proxy_set_header X-Forwarded-Host  $http_host;
         proxy_pass http://$upstream_airsonic:4040;
     }
 }