Fix: Actually rtrim() JSON in FinalNewLineNormalizer

[localheinz]

This PR

• actually appends whitespace to $json before passing it to the FinalNewLineNormalizer in a test
• actually rtrim() $json before appending PHP_EOL in the FinalNewLineNormalizer

Follows #2.

[codecov]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master     #5   +/-   ##
=======================================
  Coverage       100%   100%           
  Complexity       12     12           
=======================================
  Files             3      3           
  Lines            27     27           
=======================================
  Hits             27     27

Impacted Files	Coverage Δ	Complexity Δ
src/FinalNewLineNormalizer.php	100% <100%> (ø)	3 <0> (ø)	⬇️

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