[w3c/ServiceWorker] spec says cache should return responses with immutable headers, but no browser does that (#1456)

While working on a glitch for a chrome-specific issue, I noticed a weird behavior in all browsers.  The spec says that the responses returned from the Cache API should be immutable.  See step 5.4.2.1 here:

https://w3c.github.io/ServiceWorker/#cache-matchall

This glitch overwrites a header in a cache-returned response, though:

https://code-cache-mime-type.glitch.me/

And it appears to work in chrome, firefox, and safari.  I hit some issue in edge which prevented the service worker from installing for some reason, but I couldn't figure out why.

Given that all browsers are shipping this behavior, should we change the spec?  There is some risk of implementation now that sites could break if we start making these immutable now.

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