Fix printing with Unitful coefficient

[blegat]

This is needed for the printing in #1350 to actually work :-P

[codecov]

Codecov Report

Merging #1352 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #1352     +/-   ##
=========================================
- Coverage   89.34%   89.25%   -0.1%     
=========================================
  Files          24       24             
  Lines        3388     3647    +259     
=========================================
+ Hits         3027     3255    +228     
- Misses        361      392     +31

Impacted Files	Coverage Δ
src/print.jl	80.51% <100%> (+1.06%)	⬆️
src/variables.jl	85.88% <0%> (-2.75%)	⬇️
src/Derivatives/coloring.jl	97.57% <0%> (-0.12%)	⬇️
src/JuMP.jl	79.03% <0%> (+0.73%)	⬆️

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[mlubin]

Could you add a test for this? It's not immediately clear what the issue was.