Add `Vector` conversion

Currently there is an inconsistency between `Array` and `Vector` conversions for categorical arrays

```
julia> x = CategoricalArray([1, 1, 1, 2, 2, 3]);

julia> Array(x)
6-element Array{Int64,1}:
 1
 1
 1
 2
 2
 3

julia> Vector(x)
6-element Array{CategoricalValue{Int64,UInt32},1}:
 1
 1
 1
 2
 2
 3
```