server: Flush headers when using wait=true and stream=true
Many http clients will missbehave unless they get an initial http-
response, even when long-polling. It also saves the user/client from
having to handle headers on the first action of the watch, but rather
handle the response immediately.
Original commit: 2338481bb1d5ab3af95188d21db70512b03ccf38
From: Christoffer Vikström