[HttpFoundation] Header `HTTP_X_FORWARDED_PROTO` can contain various values

Header `HTTP_X_FORWARDED_PROTO` can contain various values. Some proxies use `ssl` instead of `https`, as well as Lighttpd mod_proxy allows value chaining (`https, http`, where `https` is always first when request is encrypted).

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Tests pass?   | yes
| Fixed tickets | #9101
| License       | MIT