Ensure flags are 0 when performing an append or prepend operation.

This change is neccessary to support libcouchbase 2.4.4.

Change-Id: Ie404c26fdc891a9ffacdbbd4f25d19b2df0f978d
Reviewed-on: http://review.couchbase.org/43867
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>
Tested-by: Brett Lawson <brett19@gmail.com>