lowercase header names

[tatethurston]

Fixes #219

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +9.20% 🎉

Comparison is base (2550b1e) 32.34% compared to head (3d9e54f) 41.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #220      +/-   ##
==========================================
+ Coverage   32.34%   41.55%   +9.20%     
==========================================
  Files          17       17              
  Lines         742      746       +4     
  Branches      131      131              
==========================================
+ Hits          240      310      +70     
+ Misses        488      422      -66     
  Partials       14       14              

Files Changed	Coverage Δ
packages/twirpscript/src/runtime/server/index.ts	80.89% <100.00%> (+43.63%)	⬆️

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