socks: Preserve buffered payload after handshake

[Snowy117]

Fixes #113 by wrapping the connection with bufio.NewCachedConn, just like the HTTP inbound:

sing/protocol/http/handshake.go

Lines 87 to 97 in 6f21f24

	if reader.Buffered() > 0 {
	buffer := buf.NewSize(reader.Buffered())
	_, err = buffer.ReadFullFrom(reader, reader.Buffered())
	if err != nil {
	return err
	}
	requestConn = bufio.NewCachedConn(conn, buffer)
	} else {
	requestConn = conn
	}
	handler.NewConnectionEx(ctx, requestConn, source, destination, onClose)