Bug 236973

When sending a request with a non-safe content type (e.g. a blob via sendBeacon), the "content-type" header is missing when it is fetched by a Service Worker.
This happens for cross-origin requests as well as for same-origin requests.

I implemented a simple test page that reproduces the issue here: https://good-breeze-16.app.baqend.com/
The test page registers a simple Service Worker which listens to fetch requests, logs the seen request headers, and does the fetch.
The test page also includes the following script tag that sends the beacon requests:
	<script>
	   // Cross Origin
           var blob = new Blob([JSON.stringify({"weather":true})], { type: "application/json" });
           navigator.sendBeacon('https://dm-clone.app.baqend.com/v1/code/endpoint?type=beaconStrange', blob);
           // Same origin
           var blob = new Blob([JSON.stringify({"weather":true})], { type: "application/json" });
           navigator.sendBeacon('https://good-breeze-16.app.baqend.com/v1/code/endpoint?type=beaconStrange', blob);
	</script>

Ways to reproduce:
1. Navigate to https://good-breeze-16.app.baqend.com/
2. Open Service Workers view (Developer -> Service Workers)
3. Refresh the page
4. Check the request headers printed in the Service Worker console and see that the "content-type" header is missing for both beacon requests.


Is there any fix for this issue?