Override Nulls.levels() instead of defining custom function

This prevents conflicts with DataArrays.

REQUIRE

@@ -1,3 +1,3 @@
 julia 0.6
 Compat 0.19.0
-Nulls 0.1.0
+Nulls 0.1.2

src/array.jl

@@ -530,7 +530,7 @@ in the data (see [`droplevels!`](@ref)).
 As a special case, `null` is never included in the levels, even if
 the array contains missing values.
 """
-levels(A::CategoricalArray) = levels(A.pool)
+Nulls.levels(A::CategoricalArray) = levels(A.pool)
 
 """
     levels!(A::CategoricalArray, newlevels::Vector; nullok::Bool=false)

src/pool.jl

@@ -173,7 +173,7 @@ function levels!(pool::CategoricalPool{S, R, V}, newlevels::Vector) where {S, R,
 end
 
 index(pool::CategoricalPool) = pool.index
-levels(pool::CategoricalPool) = pool.levels
+Nulls.levels(pool::CategoricalPool) = pool.levels
 order(pool::CategoricalPool) = pool.order
 
 isordered(pool::CategoricalPool) = pool.ordered

src/subarray.jl

@@ -1,10 +1,7 @@
 # delegate methods for SubArrays to support view
 
-for f in [:levels, :isordered]
-    @eval begin
-        $f(sa::SubArray{T,N,P}) where {T,N,P<:CategoricalArray} = $f(parent(sa))
-    end
-end
+Nulls.levels(sa::SubArray{T,N,P}) where {T,N,P<:CategoricalArray} = levels(parent(sa))
+isordered(sa::SubArray{T,N,P}) where {T,N,P<:CategoricalArray} = isordered(parent(sa))
 
 function unique(sa::SubArray{T,N,P}) where {T,N,P<:CategoricalArray}
     A = parent(sa)