Test 0x00 in header values for fetch

[JannisBush]

Currently 0x00 header values are only tested for script, iframe, and image: https://github.com/web-platform-tests/wpt/blob/master/fetch/h1-parsing/resources-with-0x00-in-header.window.js

Browser behavior regarding 0x00 in responses currently diverges for fetch, so I would suggest adding some more tests including fetching a resource with 0x00.