Looks like I did an oopsie; although API strings carry a size field, they should
still be usable as C-strings! (even though they may contain embedded NULs)
Strings that previously decoded into a msgpack special for representing
BINs with NULs now convert to Blobs. It shouldn't be possible to decode
into this special anymore after this change?
Notably, Lua strings with NULs now convert to Blobs when passed to VimL.
Reasoning is majorly the same: check whether lua has bug or API function has
bug, but on the other side: previous commit is checking whether similar bug when
using API via msgpack RPC, this commit is checking whether another API function
used via lua bindings triggers the same bug. Should additionally give a hint
about which lua code contains a bug.
