Fixes 2933: IOUtils does not depend on platform encoding any more

[tobiasbaum]

Fixes a problem that occured on platforms that had non-ASCII-based default encodings: #2933

I tested it on IBM z/OS and the problem mentioned in the issue with garbled class names disappeared. Sadly, it still doesn't fully work, but that's probably an unrelated problem, perhaps #801

Checklist

• Read the contributing guide
• PR should be motivated, i.e. what does it fix, why, and if relevant how
• If possible / relevant include an example in the description, that could help all readers
  including project members to get a better picture of the change
• Avoid other runtime dependencies
• Meaningful commit history ; intention is important please rebase your commit history so that each
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• The pull request follows coding style
• Mention Fixes #<issue number> in the description if relevant
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[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (9473470) 85.65% compared to head (f79915c) 85.65%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #2935   +/-   ##
=========================================
  Coverage     85.65%   85.65%           
  Complexity     2847     2847           
=========================================
  Files           325      325           
  Lines          8623     8623           
  Branches       1060     1060           
=========================================
  Hits           7386     7386           
  Misses          964      964           
  Partials        273      273           

Impacted Files	Coverage Δ
...main/java/org/mockito/internal/util/io/IOUtil.java	72.00% <100.00%> (ø)

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[TimvdLippe]

Thank you!