* response code is 407 proxy unauthorized, this returns the "Proxy-Authenticate" challenges.
   * Otherwise this returns an empty list of challenges.
   *
   * If a challenge uses the `token68` variant instead of auth params, there is exactly one
   * auth param in the challenge at key null. Invalid headers and challenges are ignored.
   * No semantic validation is done, for example that `Basic` auth must have a `realm`