bulkdocs response may not contain .ok property

I hit this case when syncing against Sync Gateway. First I thought it was a SG issue, but according to CouchDB docs SG is conformant.

See https://wiki.apache.org/couchdb/HTTP_Bulk_Document_API under 'Modify Multiple Documents With a Single Request' they have a sample response.

Another approach could be checking for `res.error`