Remove double base64url-encoding when generating kid

[jschlyter]

Closes #183

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.45%. Comparing base (2b735fe) to head (53fd3e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #185   +/-   ##
=======================================
  Coverage   76.45%   76.45%           
=======================================
  Files          44       44           
  Lines        4489     4489           
  Branches      841      841           
=======================================
  Hits         3432     3432           
  Misses        777      777           
  Partials      280      280           

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