Fixes #1344

[Mattriks]

• I've added and/or updated the unit tests
• I've run the regression tests
• I've built the docs and confirmed these changes don't cause new errors

This PR

• Fixes Discrete key issue #1344

Example

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(), theme1, aes[1])[1]
hstack(guides.ctxs...)

[codecov-io]

Codecov Report

Merging #1349 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1349      +/-   ##
==========================================
- Coverage   90.62%   90.59%   -0.03%     
==========================================
  Files          37       37              
  Lines        3945     3945              
==========================================
- Hits         3575     3574       -1     
- Misses        370      371       +1

Impacted Files	Coverage Δ
src/guide/keys.jl	94.73% <100%> (ø)	⬆️
src/guide.jl	90.9% <100%> (-0.26%)	⬇️

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[Mattriks]

Done.