unit tests for already resolved sort by reference issue

[jangorecki]

closes #710

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.10%. Comparing base (cf898ab) to head (38a8e89).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7348   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files          84       84           
  Lines       16128    16128           
=======================================
  Hits        15983    15983           
  Misses        145      145           

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[ben-schwen]

Unfortunately the problem still persists with other types than integer

DF1 <- data.frame(b=letters[1:3], c=c(1.1, 2.2, 3.3))
DF2 <- transform(DF1, c = c(4.4, 5.5, 6.6))
setorder(setDT(DF2), -c)