This patch ensure Headers object's associated header list
is ISO-8859-1 encoded when set using `Infra::isomorphic_encode`,
and correctly decoded using `Infra::isomorphic_decode`.
Follow-up of https://github.com/LadybirdBrowser/ladybird/pull/1893
The events tested here are decidedly async. We also can't really write
sync tests of the form "test(async () => {})". Nothing will await the
async callback.