Fix copy for pooled columns. Fixes #550

[quinnj]

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@33e5f68). Click here to learn what that means.
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@@            Coverage Diff            @@
##             master     #551   +/-   ##
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  Coverage          ?   82.61%           
=========================================
  Files             ?        7           
  Lines             ?     1455           
  Branches          ?        0           
=========================================
  Hits              ?     1202           
  Misses            ?      253           
  Partials          ?        0

Impacted Files	Coverage Δ
src/tables.jl	78.49% <ø> (ø)

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