the type of auth to create (a subtype of #SoupAuth)
the #SoupMessage the auth is being created for
the WWW-Authenticate/Proxy-Authenticate header
the new #SoupAuth, or %NULL if it could not be created
ConstructionException GTK+ fails to create the object.
Creates a new #SoupAuth of type @type with the information from @msg and @auth_header.
This is called by #SoupSession; you will normally not create auths yourself.