Can't decode str8 string

[qi7chen]

I pack a 32 bytes string in C++(using msgpack-c), and send to lua by network, but lua-cmgpack issues 'Bad data format in input' message when decoding. as MessagePack specification says, str8 type comes with a 0xd9 byte, seems not handled in lua-cmsgpack.

[slowernet]

https://github.com/antirez/lua-cmsgpack/pull/44 for this from @seppo0010