Maria, thanks for your patch!

But as I see you don't consider the case of box.NULL that has 'cdata' type, but doesn't have
__serialize metamethod.

It will lead to an error:
```
- error: '[string "return x:__serialize()"]:1: ''void *'' has no member named ''__serialize'''
...
``` 
>Четверг, 7 ноября 2019, 16:47 +03:00 от Maria <maria.khaydich@tarantool.org>:
>
>+ if type(res) == 'cdata' then
>+ results[n] = res:__serialize()
>+ end

-- 
Oleg Babin