CBQE-0: fix is_str syntax

Change-Id: I3364457a3145741b1d0399a0512fe33bc1220e48
Reviewed-on: http://review.couchbase.org/33924
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>