Add NoDiscard attributes on mutation free methods

[Baptouuuu]

Fix #18

[codecov]

Codecov Report

❌ Patch coverage is 56.52174% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.56%. Comparing base (eb11409) to head (98384ad).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/MalformedResponse/Raw.php	0.00%	3 Missing ⚠️
src/Curl.php	50.00%	2 Missing ⚠️
src/Information.php	0.00%	2 Missing ⚠️
src/MalformedResponse.php	0.00%	2 Missing ⚠️
src/Failure.php	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop      #21   +/-   ##
==========================================
  Coverage      94.56%   94.56%           
  Complexity       255      255           
==========================================
  Files             27       27           
  Lines           2134     2134           
==========================================
  Hits            2018     2018           
  Misses           116      116           

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