How to forward all requests to www subdomain including https requests?

I have a VPS with a Rails 4 application running on Ubuntu, NginX and Unicorn.

As I want all pages to be SSL encrypted, all requests to http:// are forwarded to https:// which is working fine.

This is an excerpt of my NginX configuration: <pre>
http {

    ....

    server {
            listen 80;
            server_name example.com;
            rewrite ^ https://$server_name$request_uri? permanent;
    }

    server {
            listen 443;
            server_name example.com;
            root /home/rails/public;
            index index.htm index.html;

            ssl on;
            ssl_certificate /etc/ssl/example.com.crt;
            ssl_certificate_key /etc/ssl/example.com.key;

            location / {
                    try_files $uri/index.html $uri.html $uri @app;
            }

            location @app {
                    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                    proxy_set_header X-Forwarded-Proto $scheme;
                    proxy_set_header Host $host;
                    proxy_pass http://app_server;
             }
    }

} </pre> How can I make it that all requests to http://example.com and https://example.com are forwarded to https://www.example.com?

Thanks for any help.