Creates a new #SoupAuth of type type with the information from msg and auth_header.
This is called by classSession; you will normally not create auths yourself.
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
Creates a new #SoupAuth of type type with the information from msg and auth_header.
This is called by classSession; you will normally not create auths yourself.