set forwarded-proto header for proxy targets on incoming https requests

[linki]

I run Drone behind Armor with AutoTLS enabled.

However, when enabling the Github integration in Drone it generates http:// URLs for the OAuth2 redirect URL and also for the Github Webhook URLs since it's configured to serve plain HTTP and is unaware of the proxy.

Setting the X-Forwarded-Proto header on the request to upstream if the original request was TLS fixes the problem for Drone and probably many others.

I tested the happy paths with Drone but:

• I don't know how it behaves with Upgrade requests.
• Also, if the incoming request already includes the header (e.g. from an ELB) it will be overwritten by Armor.
• Furthermore, if the upstream target is https then the header will be set as well although it wouldn't be required.

I don't think any of this leads to problems but I would love your feedback on them.

[coveralls]

Coverage decreased (-0.1%) to 13.559% when pulling 6b3763e on linki:forwarded-proto into f768850 on labstack:master.

[vishr]

@linki It looks good to me, all the cases you mentioned are valid, however they can be deferred for now. Thanks your you contribution 🎉 .