Fix tests failure

[nalimilan]

See https://github.com/nalimilan/FreqTables.jl/pull/44/files#r379828525.

Reproducer:

using NamedArrays, CategoricalArrays
NamedArray(ones(3, 2), (["a", "b", "c"], Vector(categorical([true, false]))), ("A", "B"))[1:2,:]

This is a corner case which happens when passing a Vector{<:CategoricalValue}, which gets converted to a CategoricalVector on indexing, which apparently isn't supported by NamedArrays. This only happens when crossing categorical and non categorical variables. The code should unwrap the values.