$ua->credentials( $netloc, $realm )
       $ua->credentials( $netloc, $realm, $uname, $pass )
           Get/set the user name and password to be used for a realm.

           The $netloc is a string of the form "<host>:<port>".  The username and password will only
           be passed to this server.  Example:

             $ua->credentials("www.example.com:80", "Some Realm", "foo", "secret");