fix copy not to recurse infinitely on some objects

[MichaelChirico]

Closes #4616
Closes #4617
Follow-up to #4206

I'm not totally satisfied with this solution, open to suggestions

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@bdc6da9). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #4620   +/-   ##
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  Coverage          ?   99.61%           
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  Files             ?       73           
  Lines             ?    14227           
  Branches          ?        0           
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  Hits              ?    14172           
  Misses            ?       55           
  Partials          ?        0           

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

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[mattdowle]

I can't think of a better solution currently other than moving that loop into C to avoid the dispatch to [[.person method. Since we'll be working on this area again soon, that doesn't seem worth it.
Just needs to be a comment in the code why that class attribute needs to be dropped, will do.