Fix inference of mapreduce

[timholy]

Fixes JuliaGraphics/Colors.jl#326

[codecov]

Codecov Report

Merging #103 into master will decrease coverage by 14.65%.
The diff coverage is 25%.

@@             Coverage Diff             @@
##           master     #103       +/-   ##
===========================================
- Coverage   73.72%   59.06%   -14.66%     
===========================================
  Files           1        1               
  Lines         137      171       +34     
===========================================
  Hits          101      101               
- Misses         36       70       +34

Impacted Files	Coverage Δ
src/ColorVectorSpace.jl	59.06% <25%> (-14.66%)	⬇️

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