fetch option headers difference - array of pairs

firefox (133) fetch

fetch("/path", { headers: [["A", "B"], ["C", "D"] })

results in the following headers on the request:

A: B
C: D

whatwg-fetch polyfill (3.6.20)

fetchPolyfill("/path", { headers: [["A", "B"], ["C", "D"]] })

results in the following headers on the request:

0: A,B
1: C,D
a: B
c: D