reverseproxy: Set X-Forwarded-Proto

[mholt]

This sets the X-Forwarded-Proto header to upstreams by default; seems like a good idea for wider compatibility by default; overrideable if necessary.

@brunojcm can you please give this a try? Either build from this branch or download the CI artifacts when it completes.

Closes #3275.

[bcmedeiros]

@mholt Fix is perfect!

✔ ~/git/caddy/cmd/caddy [forwarded-proto|✔]
23:24 $ ./caddy reverse-proxy --from https://public.host --to http://localhost:8081

Everything works like a charm, upstream builds redirects perfectly.

[francislavoie]

Only concern I'd have is there being an escape hatch if people don't want it passed to the backend, but setting header_up -X-Forwarded-Proto should do the trick, so 👍

[mholt]

Yep, it's the same as X-Forwarded-For in that regard.

Thanks, both!