Multi-dimensional array serialization drops shape and eltype information

When serializing a multi-dimensional array, the shape and eltype information is lost. For example

```julia
import MsgPack
matrix_a = rand(10, 10)
bytes = MsgPack.pack(matrix_a)
matrix_b = MsgPack.unpack(bytes)
```

results in:

```julia
julia> size(matrix_b)
(100,)
julia> eltype(matrix_b)
Any
```

