[semidet,multifile]http:authenticate_client(+URL, 
+Action)This hooks is called by http_open/3 
with the following Action value:
- send_auth_header(+AuthData, +Out, +Options)
 - Called when sending the initial request. AuthData contains 
the value for the http_open/3 
option 
authorization(AuthData) and Out is a 
stream on which to write additional HTTP headers. 
- auth_reponse(+Headers, +OptionsIn, -Options)
 - Called if the server replies with a 401 code, challenging the client. 
Our implementation adds a
request_header(authorization=Digest) header to Options, 
causing
http_open/3 to retry the request 
with the additional option.