Only set X-Forwarded-Proto when header not set

When the seafile docker container
is behind a proxy that does the TLS termination,
such that the seafile docker sees HTTP only,
then, it receives requests
where the following headers are set in the following way:

    X-Forwarded-Proto: https
    X-Forwarded-Ssl: on

Because the default NGINX template has this directive:

    proxy_set_header X-Forwarded-Proto $scheme

the request gets transmitted to gunicorn with the following,
contradictory values:

    X-Forwarded-Proto: http
    X-Forwarded-Ssl: on

Thus Seafile rejects the requests
with a "Contradictory scheme headers" error.

We instead change the header only when it is not set.

Fixes haiwen#226.

templates/seafile.nginx.conf.template

@@ -45,7 +45,9 @@ server {
         proxy_set_header Host $http_host;
         proxy_set_header Forwarded "for=$remote_addr;proto=$scheme";
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header X-Forwarded-Proto $scheme;
+        if ($http_x_forwarded_proto = "") {
+            proxy_set_header X-Forwarded-Proto $scheme;
+        }
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header Connection "";
         proxy_http_version 1.1;
@@ -88,7 +90,9 @@ server {
         proxy_set_header   X-Real-IP $remote_addr;
         proxy_set_header   X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header   X-Forwarded-Host $server_name;
-        proxy_set_header   X-Forwarded-Proto $scheme;
+        if ($http_x_forwarded_proto = "") {
+            proxy_set_header   X-Forwarded-Proto $scheme;
+        }
         proxy_read_timeout  1200s;
         client_max_body_size 0;