always clear AD flag in forwarding mode

It's perhaps questionable to pass AD through, but we were setting it by
mistake (!).  Fixes #98.
3 jobs from forward-clear-ad in 1 minute and 46 seconds
Status Job ID Name Coverage
  Build
passed #1137
docker linux amd64
build_job

00:00:23

 
  Test
passed #1139
docker linux amd64
deckard_job

00:00:57

passed #1138
docker linux amd64
test_job

00:00:25