Force Host header rewrite in Workhorse for Geo proxying

What does this MR do and why?

By default, the httputil reverse proxy does not change the Host header, resulting in the upstream server receiving the Host header of the current request, instead of the "expected" target (url) host.

There's an upstream "bug": https://github.com/golang/go/issues/28168

Related to #339260 (closed)

How to set up and validate locally

On a GDK + Geo setup, verify that the primary workhorse logs have host: $PRIMARY_URL instead of host: $SECONDARY_URL when accessing a proxied URL.

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