Geo secondary proxying with different URLs: Avoid getting blocked due to unexpected Host header

From !67700 (comment 645450764):

Copying the commit message here:

Avoid getting blocked due to unexpected Host header

During local testing, I found that ngrok only accepted a matching Host
header. The error response was:

Received a request for different Host than the current tunnel.
Current Tunnel: 8c921afabba4.ngrok.io
Requested Host: gdk.test

I have no idea if this is realistically only an ngrok restriction, or if we might see this behavior from other software or networking equipment.

I don't know of a downside to overwriting the Host with the remote Host, but then this area has a lot of unknown unknowns for me.

I removed the Host rewrite functionality from that MR !67700 (merged) because it seemed to cause another problem, and I wanted to let the rest of the MR get to staging.

Note: The "different URL" use-case is far less valuable than "unified URL", and does not need to block it.