minor cleanups

[raefu]

• autosave less
• reduce console spam
• avoid continuous unnecessary OPTIONS preflights (may slightly improve latency)

[LostRuins]

I think I might want to keep the content-type header - although it does work without it for kcpp, there could be backends that check or require it in order to handle the request correctly, and best practices do recommend adding it, despite the few seconds saved in the preflight if its excluded.