[w3c/ServiceWorker] content-type in csp report on fetch is missed (#1449)

When csp report initiated by browser and fetched by service worker content-type request header is missed by some reason. I created a [demo page](https://billowy-study.glitch.me/).
Being on loaded page, please open network, filter request by "report" substring and reload the page. You will see 4 requests:

- report1 xhr
- report csp
- report1 fetch
- report fetch

`report1` is called manually from js (`new XMLHttpRequest()`...), `report` - by browser because of csp rule violation.
Both xhr and csp report/report1 calls have "Content-Type: application/csp-report" provisional request header. However, only report1 fetch has such header, report1 fetch misses the Content-Type header.

Tested in latest Chrome and Firefox.



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https://github.com/w3c/ServiceWorker/issues/1449