DT[fctr] now works

[mattdowle]

Closes #1632
Supersedes #4660 (I was going to edit that PR as I usually do, but its branch was in a fork that no longer exists, plus the solution makes it work rather than make a better error, so I just did a new PR in the end.)

[codecov]

Codecov Report

Merging #5120 (35cfe3f) into master (42b492e) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 35cfe3f differs from pull request most recent head 8a1972d. Consider uploading reports for the commit 8a1972d to get more accurate results

@@           Coverage Diff           @@
##           master    #5120   +/-   ##
=======================================
  Coverage   99.38%   99.38%           
=======================================
  Files          77       77           
  Lines       14506    14506           
=======================================
  Hits        14417    14417           
  Misses         89       89           

Impacted Files	Coverage Δ
R/data.table.R	99.94% <100.00%> (ø)

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