fix(fetch): re-calculate iterator after each iteration

[KhafraDev]

Adds new WPTs from web-platform-tests/wpt@a31d3ba

Fixes 2 per-existing tests that were incorrect - I've confirmed that the behavior now matches Firefox.

Expect iterators for FormData and Headers to be much slower, as the optimizations undici did were incorrect.

From the PR itself:

In practice, this means that every iteration step needs to be performed on a Headers' sorted-and-combined headers list that should not be cached. Gecko seems to be the only engine doing this right, as both WebKit and Blink appear to carry out the entire iteration on a cached copy of the header list.

[codecov-commenter]

Codecov Report

Base: 94.11% // Head: 94.19% // Increases project coverage by +0.07% 🎉

Coverage data is based on head (6ae456e) compared to base (8d6ddb7).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1706      +/-   ##
==========================================
+ Coverage   94.11%   94.19%   +0.07%     
==========================================
  Files          53       53              
  Lines        4943     4957      +14     
==========================================
+ Hits         4652     4669      +17     
+ Misses        291      288       -3     

Impacted Files	Coverage Δ
lib/fetch/formdata.js	95.95% <100.00%> (-0.12%)	⬇️
lib/fetch/headers.js	96.57% <100.00%> (+2.09%)	⬆️
lib/fetch/util.js	85.16% <100.00%> (+1.22%)	⬆️

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