forder on Date suggest integer in verbose mode

[jangorecki]

Closes #1738
somehow it fails bmerge test with typeof INT != DATE, although I am not able to reproduce update by reference of date column

set.seed(1L)
dt = data.table(d=sample(seq(as.Date("2015-01-01"), as.Date("2015-12-31"), by="days"), 1e7, TRUE))
o = forder(dt)
str(dt)
#Classes ‘data.table’ and 'data.frame':	10000000 obs. of  1 variable:
# $ d: Date, format: "2015-04-07" "2015-05-16" ...
# - attr(*, ".internal.selfref")=<externalptr> 

[codecov]

Codecov Report

Merging #3624 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3624      +/-   ##
==========================================
+ Coverage   98.45%   98.45%   +<.01%     
==========================================
  Files          69       69              
  Lines       13284    13287       +3     
==========================================
+ Hits        13079    13082       +3     
  Misses        205      205

Impacted Files	Coverage Δ
src/forder.c	100% <100%> (ø)	⬆️

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