Add NoDiscard attributes

[Baptouuuu]

Fix #19

[codecov]

Codecov Report

❌ Patch coverage is 89.24303% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.22%. Comparing base (9d14709) to head (5fdd34d).
⚠️ Report is 3 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/Header/ContentRange.php	33.33%	4 Missing ⚠️
src/Header/Range.php	40.00%	3 Missing ⚠️
src/Header/SetCookie.php	50.00%	3 Missing ⚠️
src/Header/Host.php	33.33%	2 Missing ⚠️
src/Method.php	60.00%	2 Missing ⚠️
src/Header/Age.php	66.66%	1 Missing ⚠️
src/Header/ContentLength.php	66.66%	1 Missing ⚠️
src/Header/Cookie.php	50.00%	1 Missing ⚠️
src/Header/Date.php	50.00%	1 Missing ⚠️
src/Header/Expires.php	50.00%	1 Missing ⚠️
... and 8 more

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #21      +/-   ##
=============================================
+ Coverage      94.17%   94.22%   +0.04%     
  Complexity      1096     1096              
=============================================
  Files            197      197              
  Lines           4328     4328              
=============================================
+ Hits            4076     4078       +2     
+ Misses           252      250       -2     

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