hallenges. If the
 response code is 407 proxy unauthorized, this returns the "Proxy-Authenticate" challenges.
 Otherwise this returns an empty list of challenges.

 <p>If a challenge uses the <code>token68</code> variant instead of auth params, there is exactly
 one auth param in the challenge at key <code>null</code>. Invalid headers and challenges are
 ignored. No semantic validation is done, for example that <code>Basic</code> auth must have a <code>realm</code> auth param, this is up to the caller that 