replacement, curl only replaced the Host: header on the initial request
and didn't replace it on the following ones. This resulted in requests with
two Host: headers.
Now, curl checks if the location is on the same host as the initial request
and then continues to replace the Host: header. And when it moves to another
host, it doesn't replace the Host: header but it also doesn't make the
second Host: header get used in the request.
This change is verified by the two new test cases 184 and 185.
