fix(Headers): lowercase name in Headers.prototype.set

[KhafraDev]

Fixes a regression in a WPT from nodejs/node#43752

This is only an issue in Headers.prototype.set when using a header name that contains uppercase characters.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.90%. Comparing base (aa8782c) to head (38e0d1f).
⚠️ Report is 1961 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1535   +/-   ##
=======================================
  Coverage   94.90%   94.90%           
=======================================
  Files          50       50           
  Lines        4609     4610    +1     
=======================================
+ Hits         4374     4375    +1     
  Misses        235      235           

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[mcollina]

lgtm