ssl - Http nginx behind https ELB and index auto redirect

I've got an Amazon ELB that listens for http and https traffic. Instances behind it have nginx on port 80. Http only. So ELB forwards both https and http to http of nginx.

When I make an https request to a folder like
https://example.com/folder
it is redirected automatically to a slash version
http://example.com/folder/
but protocol becomes http. Folder contains index.html file. I assume that's what makes it work.

Is there any way to fix this? I.e. make it redirect to https instead of http. I can not enforce https globally.

My config:

http {
map $http_x_forwarded_proto $thescheme {
    default $scheme;
    https https;
}


server {
    listen 80;
    server_name example.com;
    location / {
        root /var/www/html;
        add_header X1 $scheme;
        add_header X2 $thescheme;
        index index.html;
    }
}

}

I've added X1 and X2 headers to check what protocol nginx thinks is used and if X-Forwarded-Proto header is added by ELB. X1 is http, X2 is https for the example request.

I found that adding

if (-d $request_filename) {
    rewrite [^/]$ $thescheme://$http_host$uri/ permanent;
}

inside location helps but wondering if there's a better solution.