Fix string representation of MappingNode in flow style

[semihbkgr]

fixes #622

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.36%. Comparing base (f739772) to head (021dbbe).

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #623   +/-   ##
=======================================
  Coverage   77.36%   77.36%           
=======================================
  Files          22       22           
  Lines        7885     7885           
=======================================
  Hits         6100     6100           
  Misses       1368     1368           
  Partials      417      417