Fix wrong column type for heterogeneus data

[attdona]

PR for #20

Greetings
Attilio

[codecov]

Codecov Report

Merging #21 (9ab3c12) into main (272624b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #21   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files           1        1           
  Lines          95       95           
=======================================
  Hits           87       87           
  Misses          8        8           

Impacted Files	Coverage Δ
src/JSONTables.jl	91.57% <100.00%> (ø)

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[quinnj]

Thanks @attdona! Sorry for the slow review here; this is great!