fix: keep raw header name

[tsctx]

Follow up of #3160.
In the following cases. Lose the original raw header name.

await fetch(new Request(url, { headers: [['Content-Type', 'text/plain']] }), { method: 'GET' })

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.17%. Comparing base (e009b0c) to head (8c33986).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3183   +/-   ##
=======================================
  Coverage   94.17%   94.17%           
=======================================
  Files          90       90           
  Lines       24434    24437    +3     
=======================================
+ Hits        23011    23014    +3     
  Misses       1423     1423           

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[mcollina]

lgtm