Add Vary: X-Forwarded-Proto for https redirects

From mark's comments on https://gerrit.wikimedia.org/r/#/c/86268/, we
should be setting Vary: X-Forwarded-Proto when the response varies on
the protocol, which is the case for redirects from http->https.

Change-Id: Ie6dc9da4e804199b99e3a7abbea7775a181801a0
Merge "Add Vary: X-Forwarded-Proto for https redirects"