fix: normalize fetch headers

[pi0]

resolves #81

Fixing couple of issues to normalize headers (lowercase and remove undefined values). CI seems fixing (locally it was working for me on 16 too not sure why...)

[codecov[bot]]

Codecov Report

Merging #82 (02ebb22) into main (af23a7d) will increase coverage by 0.65%. The diff coverage is 74.62%.

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   70.00%   70.65%   +0.65%     
==========================================
  Files           6        6              
  Lines         310      426     +116     
  Branches       42       48       +6     
==========================================
+ Hits          217      301      +84     
- Misses         93      125      +32     

Impacted Files	Coverage Δ
src/registry.ts	35.00% <29.16%> (-4.14%)	:arrow_down:
src/providers.ts	44.11% <33.33%> (-2.44%)	:arrow_down:
src/_utils.ts	74.43% <81.53%> (+4.66%)	:arrow_up:
src/giget.ts	82.46% <89.04%> (+1.59%)	:arrow_up:
src/types.ts	100.00% <100.00%> (ø)

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