the #SoupMessage auth is being updated for
the WWW-Authenticate/Proxy-Authenticate header
true if auth is still a valid (but potentially unauthenticated) #SoupAuth. false if something about auth_params could not be parsed or incorporated into auth at all.
Updates auth with the information from msg and auth_header, possibly un-authenticating it.
As with soup.auth.Auth.new_, this is normally only used by classSession.