[w3c/ServiceWorker] NavigationPreloadManager.setHeader spec should verify valid HTTP header values (#1474)

The [corresponding WPTs](https://github.com/web-platform-tests/wpt/blob/b52533c5a60cbd0902dc19d325ab1eb594b59138/service-workers/service-worker/navigation-preload/get-state.https.html#L76) expect `setHeader` to reject when called with a value containing the null byte or HTTP newline bytes. It's unclear whether `setHeader` should also prohibit or trim leading and trailing whitespace. The Fetch standard defines a [HTTP header value](https://fetch.spec.whatwg.org/#concept-header) as having no leading or trailing whitespace, though.

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