Nginx – proxy_set_header x-forwarded-proto Not Working

Since i am a beginner in nginx proxying i have a question.

The gist of it:
Where does an nginx proxy set defaults for php's
$_SERVER["HTTP_X_FORWARDED_HOST"],
$_SERVER["HTTP_X_FORWARDED_SERVER"] and
$_SERVER["HTTP_X_FORWARDED_FOR"]?

More Info:
This is a development environment.
I have a vhost entry on my nginx proxy:

server {
    listen 192.168.1.17:443 ssl;
    server_name foo.bar.com;

    ssl_certificate /etc/nginx/ssl/boerse.de.crt;
    ssl_certificate_key /etc/nginx/ssl/boerse.de.key;

    location / {
        proxy_pass http://foo_cluster/;

        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-Host $host;
        proxy_set_header X-Forwarded-Server $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

foo_cluster only upstreams one server:

upstream foo_cluster {
    ip_hash;
    server 192.168.5.33:80 fail_timeout=30s;
    server 192.168.5.34:80 fail_timeout=30s down;
}

On my apache webserver at 192.168.5.33 i have another vhost:

<VirtualHost *:80>
    ServerName foo.bar.com
    ServerAdmin webmaster@bar.com
    DocumentRoot /var/www/foo-bar-com
    <Directory /var/www/foo-bar-com/>
            Options Indexes FollowSymLinks MultiViews
            AllowOverride ALL
            Order allow,deny
            allow from all
    </Directory>

    LogLevel warn
    ErrorLog /var/log/apache2/foo.bar.com_error.log
    CustomLog /var/log/apache2/foo.bar.com_access.log combined

    ServerSignature On
    SetEnv ApplicationConfigFiles "/var/ApplicationConfigFiles/"

    php_value include_path ".:/var/www/baz/global/php/base:/var/www/foo-bar-com/vendor/library/:/var/www/foo-bar-com/vendor/models/model/:"
    php_value auto_prepend_file /var/www/foo-bar-com/class/functions/prepend.php
    php_value auto_append_file /var/www/foo-bar-com/class/functions/append.php
</VirtualHost>

In my hosts file on my local machine i have added the server:

...
192.168.1.17 foo.bar.com
...

In my prepend.php i just output superglobal $_SERVER and stop:

<?php
    var_dump($_SERVER); exit;
    ...

Alright. Now with all that set up i open the wesite on my browser at https://foo.bar.com

Here an excerpt of the output source code:

array(32) {
    ...
    ["HTTP_HOST"]=>
    string(11) "foo.bar.com"
    ["HTTP_X_FORWARDED_HOST"]=>
    string(11) "foo.bar.com"
    ["HTTP_X_FORWARDED_SERVER"]=>
    string(11) "foo.bar.com"
    ["HTTP_X_FORWARDED_FOR"]=>
    string(13) "192.168.2.131"
    ["HTTP_CONNECTION"]=>
    string(5) "close"
    ...

Since i am missing expected X-Forwarded-Proto i comment out all proxy_set_header in the nginx vhost file (and reload the service):

server {
    listen 192.168.1.17:443 ssl;
    server_name foo.bar.com;

    ssl_certificate /etc/nginx/ssl/boerse.de.crt;
    ssl_certificate_key /etc/nginx/ssl/boerse.de.key;

    location / {
        proxy_pass http://foo_cluster/;

        #proxy_set_header Host $host;
        #proxy_set_header X-Forwarded-Host $host;
        #proxy_set_header X-Forwarded-Server $host;
        #proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        #proxy_set_header X-Forwarded-Proto $scheme;
    }
}

Yet, the result output remains the same.

I have searched google for why this might be the case all day.
nginx documentation for proxy_set_header states:

This directive allows to redefine and to add some request header lines which will be transferred to the proxied server.

I wonder why it says exactly "redefine". I have found no answer as to what is predefined here to be redefined via proxy_set_header.

I have searched /etc/nginx/nginx.conf for proxy_set_header, in fact i greped the whole /etc/nginx directory with

grep -ri x-forwarded-host *

Only results found where in sites-available and sites-enabled.

I searched the same on the apache webserver with no useful results (i thought they may be set in php code but i was wrong).

I am doubtful as to whether my nginx vhost file is even the used one.

Thank you for your time.

Tldr:
Where does an nginx proxy set defaults for php's
$_SERVER["HTTP_X_FORWARDED_HOST"],
$_SERVER["HTTP_X_FORWARDED_SERVER"] and
$_SERVER["HTTP_X_FORWARDED_FOR"]?