Modify a url by first parsing it and then replacing components with the non-NULL arguments of this function.

modify_url(url, scheme = NULL, hostname = NULL, port = NULL,
  path = NULL, query = NULL, params = NULL, fragment = NULL,
  username = NULL, password = NULL)



the url to modify

scheme, hostname, port, path, query, params, fragment, username, password

components of the url to change