msgpack encode sparse array as map

[rtokarev]

When I use mspack.encode/decode to pass Lua tables between Lua scripts I expect that the encoded table after decoding will be exactly the same. But this isn't always true. When the table represents a sparse array msgpack.encode could serialize it to msgpack array, and after decoding in another Lua script my table will consist mspack.NULL's that not the same as Lua nil's.

unix/:/var/run/tarantool/xtaz_1.control> t = { [1] = 1, [3] = 3 }
---
...

unix/:/var/run/tarantool/xtaz_1.control> tt = {} for _, n in pairs(t) do table.insert(tt, n) end
---
...

unix/:/var/run/tarantool/xtaz_1.control> tt
---
- - 3
  - 1
...

unix/:/var/run/tarantool/xtaz_1.control> t = msgpack.decode(msgpack.encode((t)))
---
...

unix/:/var/run/tarantool/xtaz_1.control> t
---
- [1, null, 3]
...

unix/:/var/run/tarantool/xtaz_1.control> tt = {} for _, n in pairs(t) do table.insert(tt, n) end
---
...

unix/:/var/run/tarantool/xtaz_1.control> tt
---
- - 1
  - null
  - 3
...

unix/:/var/run/tarantool/xtaz_1.control>

I know that there is a hint in metatable (__serialize = 'map') that force msgpack.encode to always serialize table as a msgpack map, but in my opinion, it's better to serialize sparse array as a map by default and serialize it as an array using the hint. It's more expected behavior rather than trying to guess.