Avoid StreamOutputRange in the WebSocket send implementation

[s-ludwig]

This causes an additional write() call on the underlying connection stream (which could be a TLS stream) for large frame payloads (starting from ~252 bytes). Since OutgoingWebSocketMessage already uses an Appender to accumulate payload data, it could easily reserve a few bytes for the header, so that the full frame could be sent in one go.

Original discussion: https://forum.rejectedsoftware.com/groups/rejectedsoftware.vibed/thread/43534/