PYCBC-138: Fix incr_multi/decr_multi

Change-Id: I9a86465429b482c35d839c09bb05285adf20aa32
Reviewed-on: http://review.couchbase.org/26986
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>