Fix builtin changes filters with ?descending=true
Fix builtin changes filters with ?descending=true

After a recent optimization (COUCHDB-1288), querying the _changes
feed using one of the builtin filters (_doc_ids, _design) with
?descending=true was not producing the correct results anymore.
This changes fixes it and adds a test.

smithsz/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,KlausTrainer/couchdb-couch,ngpestelos/couchdb-couch,ngpestelos/couchdb-couch,apache/couchdb-couch,KlausTrainer/couchdb-couch,apache/couchdb-couch,ngpestelos/couchdb-couch,cloudant/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,alanning/couchdb-couch,cloudant/couchdb-couch,smithsz/couchdb-couch,fkaempfer/couchdb,KlausTrainer/couchdb-couch,fkaempfer/couchdb,alanning/couchdb-couch,alanning/couchdb-couch,smithsz/couchdb-couch