zero out flags for append/prepend

This ensures that we don't get an odd error from the library when
setting flags, since the flags on the server side are ignored.

Change-Id: I21ac9373684a2b644516ef5b83d60237071f7ed2
Reviewed-on: http://review.couchbase.org/43581
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>