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 WebIDL for the `Headers` constructor specifies that the `init`
parameter is optional and must be of type `HeadersInit`. While the
parameter can be omitted (or explicitly set to `undefined`),
`null` is not a valid value.
This change fixes at least 2 "Create headers with null should throw"
WPT subtests which I have imported in this patch.