ensure correct ordering in set_units.mixed_units

[Enchufa2]

Fixes #271, and adds a test case to avoid future regressions.

[codecov]

Codecov Report

Merging #272 (79ebaa1) into master (2512a42) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
+ Coverage   94.14%   94.16%   +0.02%     
==========================================
  Files          18       18              
  Lines         871      874       +3     
==========================================
+ Hits          820      823       +3     
  Misses         51       51              

Impacted Files	Coverage Δ
R/mixed.R	98.11% <100.00%> (+0.11%)	⬆️

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