Include prefer wait in watch request even after initial 404 read request

[jfallows]

Description

Before this change, if the initial request for zilla.yaml receives 404 not found, then the next watch request would not include prefer: wait=N so it can return immediately with 404 also, causing the next watch request to be sent only after a poll interval, which is 60s by default.

After this change, even if the initial request for zilla.yaml receives 404, then the next watch request now includes prefer: wait=N and responds immediately when the response becomes available, with no need for polling.