Prevent X-Forwarded-Proto forward during external auth subrequest

[wayt]

What this PR does / why we need it:
This prevents X-Forwarded-Proto forwarding during external auth sub request.

This cause external auth to return 308 or 301 even if its target url is an hardcoded https.

Which issue this PR fixes
fixes #3389

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[wayt]

/assign @ElvinEfendi

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, wayt

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[aledbf]

@wayt please sign the CLA

[wayt]

@aledbf I'm having an issue with the Linux Foundation process, I've contacted them by email to solve this.

[wayt]

I signed it

[ElvinEfendi]

@wayt I think you have a typo here, it should be X-Forwarded-Proto

[wayt]

.... yup