Add support for oauth2 plugin X-Forwarded-Proto HTTPS override

[CyExy]

We have an SSL wrapper and load balancer in front of kong so it would be unnecessary to wrap it just for kong.

"a de facto standard for identifying the originating protocol of an HTTP request, since a reverse proxy (load balancer) may communicate with a web server using HTTP even if the request to the reverse proxy is HTTPS ..."

source https://en.wikipedia.org/wiki/List_of_HTTP_header_fields#Common_non-standard_request_fields

[CyExy]

Not sure why the CI failed, runned make test-all locally and all of the test pass.

[subnetmarco]

@CyExy Looks good to me. Can you submit the PR against the next branch?

Tests are failing because of a problem we currently have with our CI systems, but if it works locally it's good.

[CyExy]

@thefosk hoped to sneak the fix in as a hot fix :) but if you would prefer to release it next I can rebase against next branch in a new PR.

[thibaultcha]

Thanks! We are going to release a patch version for this and #644.

[CyExy]

Thank you very much!