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 Response interface of the Fetch API can now parse form urlencoded bodies when Content-Type is set to 'application/x-www-form-urlencoded'.