[fix] replace X-Scheme by X-Forwarded-Proto header (#5107)

The HTTP X-Forwarded-Proto (XFP) request header is a *de-facto* standard header
for identifying the protocol (HTTP or HTTPS) that a client used to connect to a
proxy or load balancer.[1]

The ``X-Scheme`` header was added 10 years ago, why ``X-Scheme`` was used back
then and not ``X-Forwarded-Proto``, nobody knows today / possibly because
``X-Forwarded-Proto`` wasn't a *de-facto* standard back then.

[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/X-Forwarded-Proto
[2] 6ef7c3276

searx/flaskfix.py

@@ -21,7 +21,7 @@ class ReverseProxyPathFix:
         proxy_pass http://127.0.0.1:8000;
         proxy_set_header Host $host;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header X-Scheme $scheme;
+        proxy_set_header X-Forwarded-Proto $scheme;
         proxy_set_header X-Script-Name /myprefix;
         }