Foverlaps bug fix for overlapping on POSIXct objects < 1970-01-01

[arunsrinivasan]

foverlaps returned incorrect results when overlapping on POSIXct dates with datetime values < 1970-01-01. closes #3349

[arunsrinivasan]

@mattdowle added you just to make sure I've created the PR as expected. Committing after a while ..

[codecov]

Codecov Report

Merging #3382 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3382      +/-   ##
==========================================
- Coverage   95.09%   95.09%   -0.01%     
==========================================
  Files          65       65              
  Lines       12122    12120       -2     
==========================================
- Hits        11528    11526       -2     
  Misses        594      594

Impacted Files	Coverage Δ
R/foverlaps.R	100% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #3382 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #3382      +/-   ##
==========================================
- Coverage   95.09%   95.09%   -0.01%     
==========================================
  Files          65       65              
  Lines       12122    12120       -2     
==========================================
- Hits        11528    11526       -2     
  Misses        594      594

Impacted Files	Coverage Δ
R/foverlaps.R	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5cd3a46...14e011a. Read the comment docs.