Can't round-trip integer CategoricalArrays

```jl
julia> using CategoricalArrays, DataFrames, Arrow
julia> Arrow.write("/tmp/my.arrow",DataFrame(x=CategoricalArray([1,2,3])))
"/tmp/my.arrow"

julia> DataFrame(Arrow.Table("/tmp/my.arrow")).x|>eltype
Int64

[69666777] Arrow v2.3.0
```