Allow relative urls when parsing

[hadley]

Fixes #449

[hadley]

Should also add:

#' # Use a relative url
#' req <- request("http://example.com/a/b/c")
#' req |> req_url_relative("..")


#' @export
#' @rdname req_url
req_url_relative <- function(req, url) {
  check_request(req)

  new_url <- url_parse(url, base_url = req$url)
  req_url(req, url_build(url))
}