CBQE-2450:n1ql fix string fns

Change-Id: Id730e0a9ef8e23f369d99198fc4c222d96ac7a1e
Reviewed-on: http://review.couchbase.org/42619
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>