Improve testJSONMarshal

[exageraldo]

Fixes: #2699

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.27%. Comparing base (bcfcb89) to head (5eb1677).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3519   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files         183      183           
  Lines       16099    16099           
=======================================
  Hits        14695    14695           
  Misses       1230     1230           
  Partials      174      174           

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