[w3c/ServiceWorker] should Cache API Vary matching use combined header values? (#991)

At some point the fetch spec was changed such that `Headers.get()` now returns combined values.  The SW spec was using `Headers.get()` for Vary header matching previously and I don't think it was updated to account for this change.

What is the intention here?  Should Cache API vary matching be performed on combined values (new fetch `get()`) or first header value (old fetch `get()`)?

@jakearchibald @annevk @inexorabletash 

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