couch-doc-update now also works for _security

It also clears doc fields that are not set in the new data when updating a doc.
Revert "don't use couchdb::document for creating _security, cause this special doc doesn't have an _id (#3706)"

This reverts commit dacaeb4d98be9468336923f5501822d389bda768.

couch-doc-update and thus couchdb::document now also work for _security
documents. We simply add the given _id when none is present in the
existing doc. Using the normal saving of CouchRest::Document then works.
use require => rather than the -> syntax
Merge branch 'feature/3709-check-security-before-overwriting'