Do not send credentials when they are None

CouchDB in admin party returns a 401. This does not seem to occur for everyone, but I do not understand why not. Perhaps not much admin party use is around, but I do use it for some testing.

The issue seems obvious, inescapable, and immediately reproducible.

See https://github.com/apache/couchdb/blob/master/src/couchdb/couch_httpd_auth.erl#L55
https://github.com/apache/couchdb/blob/master/src/couchdb/couch_httpd_auth.erl#L66