fix: correctly handle multi-value rawHeaders in fetch

[mcollina]

Fixes #4389. When an interceptor is used, rawHeaders can contain arrays for multi-value headers like Set-Cookie. This fix ensures each value is appended individually to the HeadersList.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 83.33333% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.93%. Comparing base (5145a7c) to head (f3fcb85).

Files with missing lines	Patch %	Lines
lib/web/fetch/index.js	83.33%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4938      +/-   ##
==========================================
- Coverage   92.93%   92.93%   -0.01%     
==========================================
  Files         112      112              
  Lines       35709    35725      +16     
==========================================
+ Hits        33187    33201      +14     
- Misses       2522     2524       +2     

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