NullableCategoricalArrays constructor and levels ordering

[cjprybol]

The constructor for NullableCategoricalArray does not adhere to the ordering of the data when ordering levels.

julia> using CategoricalArrays
WARNING: Method definition ==(Base.Nullable{S}, Base.Nullable{T}) in module Base at nullable.jl:244 overwritten in module NullableArrays at /Users/Cameron/.julia/v0.6/NullableArrays/src/operators.jl:128.

julia> x = levels!(CategoricalArray(["B", "B", "A", "A"]), ["C", "B", "A"])
4-element CategoricalArrays.CategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(x)
3-element Array{String,1}:
 "C"
 "B"
 "A"

julia> nullx = NullableCategoricalArray(x)
4-element CategoricalArrays.NullableCategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(nullx) # why did the levels change?
3-element Array{String,1}:
 "A"
 "B"
 "C"

julia> nullx = NullableCategoricalArray(x, ordered=true)
4-element CategoricalArrays.NullableCategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(nullx) # still reordered even with ordered=true
3-element Array{String,1}:
 "A"
 "B"
 "C"

julia> droplevels!(nullx) # does not reset the order
4-element CategoricalArrays.NullableCategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(nullx)
2-element Array{String,1}:
 "A"
 "B"

same for Array{Strings}

julia> y = ["B", "B", "A", "A"]
4-element Array{String,1}:
 "B"
 "B"
 "A"
 "A"

julia> nully = NullableCategoricalArray(y)
4-element CategoricalArrays.NullableCategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(nully) # ordering
2-element Array{String,1}:
 "A"
 "B"

julia> nully = NullableCategoricalArray(y, ordered=true)
4-element CategoricalArrays.NullableCategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(nully) # ordering
2-element Array{String,1}:
 "A"
 "B"

julia> droplevels!(nully)
4-element CategoricalArrays.NullableCategoricalArray{String,1,UInt32}:
 "B"
 "B"
 "A"
 "A"

julia> levels(nully)
2-element Array{String,1}:
 "A"
 "B"

[nalimilan]

I have a fix here, the hardest part is writing comprehensive tests to cover all cases.

[cjprybol]

Awesome! Looking forward to reviewing the code.

[nalimilan]

OK, see #63.