If there is an old digest and a new digest was sent with the stale
directive then the new digest replaces the old digest.
Prior to this change the stale directive was ignored and a new digest
would cause error CURLE_BAD_CONTENT_ENCODING in any case.
- If the server has provided another challenge use it as the replacement
input token if stale=TRUE. Otherwise previous credentials have failed
so return CURLE_LOGIN_DENIED.
Prior to this change the stale directive was ignored and if another
challenge was received it would cause error CURLE_BAD_CONTENT_ENCODING.