(jwk.Set).Clone, more robust test, fix another off by one

[lestrrat]

fixes #414

[codecov-commenter]

Codecov Report

Merging #415 (3acfec5) into main (5e2de6e) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #415      +/-   ##
==========================================
+ Coverage   69.57%   69.61%   +0.04%     
==========================================
  Files          80       80              
  Lines        9180     9186       +6     
==========================================
+ Hits         6387     6395       +8     
+ Misses       1944     1942       -2     
  Partials      849      849              

Impacted Files	Coverage Δ
jwk/set.go	83.67% <100.00%> (+3.23%)	⬆️

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