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_new(), this is normally only used by #SoupSession.