When front-end nginx (through https:443) forward to apache2 (through htt...

According to some use-cases with apache2/nginx proposes client resource through http instead of https. This includes all `href` and `src` ressources. Web browser, by default, block (security reasons) theses ressources.

As [asked](https://github.com/SSilence/selfoss/pull/582#issuecomment-58396733) I put the changes (lighter btw) at the right place.

Note that nginx front-end users need to location as this:
````
      location / {
         proxy_set_header X-Nginx-Scheme $scheme;
          proxy_set_header X-Forwarded-Port $server_port;
          proxy_pass   http://<whatever IP you need>:<whatever port you need>/;
          include      /etc/nginx/proxy.conf;
      }
````
I think it good to pull now :) :bug: (otherwise, just ask)