CBQE-0: use .get in case dict key missing

causes attempt to get postcondition method to fail

Change-Id: Ia221491fdf3fa3b707728cbd2f74524c261dcfa2
Reviewed-on: http://review.couchbase.org/25408
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>