Hello.

Issue

We used to control the scheme of the URL with an environment variable (HTTP_SCHEME).
It used to work fine until Symfony 3.3.7

Reproducer

We noticed a regression in the processing of environment variables and routing schemes.

I have created a producer here.
It's an extra commit on top of the standard edition with Symfony 3.3.*

I order to reproduce the issue you need a real web server (does not work
with PHP -S, nor bin/console server:run).

In the web server you have to add the following line in the PHP location:

fastcgi_param HTTP_SCHEME https;

edit: read the next comment

Results

(I removed useless headers )

With Symfony 3.3.6:

>[{..}/dev/debug/sf-routing-schema-param](master *) curl http://sf-routing-schema-param.localhost:8888/app_dev.php -I
HTTP/1.1 301 Moved Permanently
Location: https://sf-routing-schema-param.localhost/app_dev.php/

With Symfony 3.3.7, 3.3.8, and 3.3.*@dev:

>[{..}/dev/debug/sf-routing-schema-param](master *) curl http://sf-routing-schema-param.localhost:8888/app_dev.php -I
HTTP/1.1 200 OK
Server: nginx/1.12.1