Table with string number as key is mistreated as array.

[yihuang]

local mp = require 'cmsgpack'
local tbl = {['1']=1}
local s = mp.pack(tbl)
print('length:', #s)
print('first byte:', string.byte(s, 1))

The table {['1']=1} is encoded to \x90, which is for [].

The script above output like this:

length: 1
first byte: 144