This could have corrupted requests where multiple headers had the same name,
    as in cookies.
 *  Fix: Reject bad characters in the URL hostname. Previously characters like
    `\0` would cause a late crash when building the request.
 *  Fix: Allow interceptors to change the request method.
 *  Fix: Don’t use the request's `User-Agent` or `Proxy-Authorization` when

    Builder<String, Integer> builder =
        new Builder<String, Integer>()
            .put("one", 1)
            .put("one", 1); // throwing on this line might be better but it's too late to change

    try {
      builder.buildOrThrow();
      fail();
    } catch (IllegalArgumentException expected) {
    }
  }

  public void testBuildKeepingLast_allowsOverwrite() {