Fix stale=true on digest requests with unknown nonce

Fix stale=true on digest requests with unknown nonce

The nonce staleness check only worked if the stale nonce had not yet
been garbage collected, often resulting in incorrect stale=false
responses and resulting auth popups when using digest auth.

Note: this fix is different from how it's done in squid-2 where fixHeader
is called on all schemes in such conditions but only the active one with
and auth_user_request. Not entirely sure why that is done, but commit
message says something about Negotiate authentication.