Discrete key issue

[Mattriks]

In this example, the key with 3 columns is not drawable:

datas = Gadfly.Data(color=rand(["A","B","C","D"], 100))
scales = [Scale.color_discrete(levels=["A","B","C","D"])]
aes = Scale.apply_scales(scales, datas)
theme1 = Theme(key_max_columns=4)
guides = render(Guide.colorkey(title="Colors"), theme1, aes[1])[1]
# draw(PNG(), hstack(guides.ctxs[1:4]...)) # fails
draw(PNG(), hstack(guides.ctxs[[1,2,4]]...)) # works

I'll submit a PR to fix this.