[ServiceWorker] should Cache support VARY:*,otherheader or only VARY:* (#656)

Right now it seems we support VARY:*,otherheader in step 9.5.1 here:

  https://slightlyoff.github.io/ServiceWorker/spec/service_worker/index.html#query-cache-algorithm

Here we continue looking at further VARY header values to see if there is a conflict.

In [RFC7231][], though, it states:

  The value consists of either a single asterisk ("*") or a list of header field names

Further, gecko's network stack allowed VARY:*,otherheader, but immediately considers the Response a match if it sees a * value.  (As opposed to continuing to look at other header values in SW spec.)

What does blink do?

Should we strictly match [RFC7231][]?  Or if we want web compat, should we match what browsers (gecko at least) are doing today by treating * as an auto-match even if there are other VARY headers?

@jakearchibald 

[RFC7231]: https://tools.ietf.org/html/rfc7231#section-7.1.4

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https://github.com/slightlyoff/ServiceWorker/issues/656