fix CategoricalArrays #330

[PharmCat]

I don't know is it bug or feature, but this code doesn't work with 0.7.8:

using StatsModels, CategoricalArrays
data = (a = [1,2,3], b = ["2", "2", "3"], c=categorical([1,2,3]))
c1 = Dict(:a => StatsModels.FullDummyCoding())
c2 = Dict(:b => StatsModels.FullDummyCoding())
c3 = Dict(:c => StatsModels.FullDummyCoding())
StatsModels.schema(data, c1) # Fine
StatsModels.schema(data, c2) # Fine
StatsModels.schema(data, c3) # ERROR

with this fix it seems work... see #330 for details. If this PR is not relevant - just delete.

[palday]

Suggested change

	ContrastsMatrix(Matrix(1.0I, length(levels), length(levels)), Vector(levels), Vector(levels), C)
	ContrastsMatrix(Matrix(1.0I, length(levels), length(levels)), collect(levels), collect(levels), C)

[PharmCat]

seems collect doesn't work with Categorical 1.0