Correctly check content type

[jschlyter]

Current we log a warning if we receive Content-Type: application/json; charset=utf-8, but want application/json. This PR fixes this.

N.B. a charset option for JSON content is moot, since only utf8 may be used.

[codecov-commenter]

Codecov Report

Merging #114 (8ef4566) into main (366d889) will decrease coverage by 0.16%.
The diff coverage is 93.02%.

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
- Coverage   77.59%   77.42%   -0.17%     
==========================================
  Files          41       41              
  Lines        4342     4328      -14     
  Branches      841      838       -3     
==========================================
- Hits         3369     3351      -18     
- Misses        709      712       +3     
- Partials      264      265       +1     

Impacted Files	Coverage Δ
src/cryptojwt/key_bundle.py	79.20% <92.30%> (-0.66%)	⬇️
src/cryptojwt/utils.py	82.31% <100.00%> (+0.49%)	⬆️
src/cryptojwt/key_issuer.py	80.25% <0.00%> (-0.95%)	⬇️

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