fix(headers): iterate over cloned list

[lukeed]

This matches the browser behavior, example:

let demo = new Headers({ foo: '123', bar: '456' });

for (let [k,v] of demo) { 
  demo.delete(k);
  demo.set(`x-${k}`, v);
}

[...demo];
//=> [
//=>   ['x-bar', '456'],
//=>   ['x-foo', '123'],
//=> ]

Note: This is included as a test

[codecov-commenter]

Codecov Report

Merging #1081 (a6c8b23) into main (2fadc07) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1081   +/-   ##
=======================================
  Coverage   95.09%   95.09%           
=======================================
  Files          39       39           
  Lines        3750     3753    +3     
=======================================
+ Hits         3566     3569    +3     
  Misses        184      184           

Impacted Files	Coverage Δ
lib/fetch/headers.js	100.00% <100.00%> (ø)

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[ronag]

If you could also find ref to spec that would be great.

[lukeed]

From the last sentence in this section

The value pairs to iterate over are the return value of running sort and combine with this’s header list.

Where sort and combine is an instruction-set for creating a new list.

[lukeed]

Adding another test to this PR, which is also failing in main branch

[mcollina]

lgtm

[Ethan-Arrowood]

LGTM!

[Ethan-Arrowood]

The infinite iteration test is a good check as well. Totally missed that; I wonder if something like that would be valuable for the web platform tests