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.