/usr/lib/swipl/library/ext/http/http/http_open.pl
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 http_set_authorization(+URL, +Authorization) is det
Set user/password to supply with URLs that have URL as prefix. If Authorization is the atom -, possibly defined authorization is cleared. For example: 
?- http_set_authorization('http://www.example.com/private/',
                          basic('John', 'Secret'))
To be done
- Move to a separate module, so http_get/3, etc. can use this too.