Uncessarily setting Host when requesting redeem URL

[castlemilk]

Describe the bug When making a request to the internal auth URL for the /redeem endpoint, the Host header for the request is being set to be the external URL. This is causing issues with routing tools like Istio, where the Host header will be used for routing decisions and should represent the intended destination (in this case the internal auth hostname).

https://github.com/buzzfeed/sso/blob/12dc0fdcfe6bcc95d3f67b91b30030fd8c42abfc/internal/proxy/providers/sso.go#L92

Expected behavior Host header should should be either not overwritten or represent the internal auth endpoint.

version: 1.2.0