unique() returns levels, not the CategoricalArray

[alyst]

It's strange that I have realized it only now, but unique(v::CategoricalArray{S}) returns Vector{S}, and not the categorical array of the same type as v.
Should it actually preserve the type of v to conform to the Base.unique() definition?

Otherwise some user code that should work regardless of the vector type (e.g. Vector{String} or CategoricalVector{String}) produces errors.

[bkamins]

This is consistent with the docstring of unique for CategoricalArray:

Return levels which appear in A in their order of appearance. This function is significantly slower than levels since it needs to check whether levels are used or not.

However, indeed, it is strange and I would also expect CategoricalArray.

[nalimilan]

See previous discussion at #129. The current behavior of unique is indeed a bit strange, but can you show in what cases it's a problem?