CBQE-2448: n1ql: update keys syntax

Change-Id: I5e3acb13f4f85560b8c0007e55ee260dca04ee0e
Reviewed-on: http://review.couchbase.org/42569
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>