Fix unboxed serialization including constructor ID
Fix unboxed serialization including constructor ID

This only affected FutureSalts, but it's useful that it behaves
the way it should. cc @JuanPotato for spotting the bug.

expectocode/Telethon,LonamiWebs/Telethon,LonamiWebs/Telethon,LonamiWebs/Telethon,LonamiWebs/Telethon