when as.data.frame dispatches to list method, force input to list

[MichaelChirico]

Closes #4526

I'm not sure if there's an efficiency implication to the as.list but it feels right to do regardless.

[codecov]

Codecov Report

Merging #4529 (be116e5) into master (16df423) will not change coverage.
The diff coverage is 100.00%.

❗ Current head be116e5 differs from pull request most recent head 84efb7e. Consider uploading reports for the commit 84efb7e to get more accurate results

@@           Coverage Diff           @@
##           master    #4529   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          73       73           
  Lines       14448    14448           
=======================================
  Hits        14365    14365           
  Misses         83       83           

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

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[MichaelChirico]

thanks! looking better