Add tests for JWTError and JWTHeader+CommonFields

[fpseverino]

Issue #138

• Add test for JWTError description
• Add test for JWTHeader common fields

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.28%. Comparing base (81be268) to head (2a77c83).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
+ Coverage   78.86%   83.28%   +4.42%     
==========================================
  Files          60       60              
  Lines        1424     1424              
==========================================
+ Hits         1123     1186      +63     
+ Misses        301      238      -63     

see 6 files with indirect coverage changes