Editor: do not remove id field for docs

COUCHDB-2341
Editor: don't replace custom id attribute with document-id

If we have a document with an id attribute "foo" and _id: "customid"
added from the user and we do a PUT request for an update,
CouchDB will return: {"ok":true, id: "customid", rev: "123"}

The Model will think it has the id "customid" instead of "foo"