[Test-Proxy] Handle `Host` header properly

1. We should be taking the `host` header as-is if the target is not the localhost address (EG the co-opted test-proxy endpoint)
2. If the user provides `x-recording-upstream-host-header` we should honor that
3. In all cases, It should still be recorded (it is not currently)

It is a discussion as to whether or not we want to just ignore a bad host header set targeting the test-proxy localhost address.