Re: [whatwg/xhr] Web compat implications of making getAllResponseHeaders lowercase (#146)

FWIW I agree with the developing consensus here that we should continue with the change as planned, based on:

- No user complaints were reported during either Chrome 60 beta or Safari 11 beta process suggesting (in concert with the other compat analysis) that the total user impact is quite low ([Minimizing end-user impact](https://docs.google.com/document/d/1RC-pBBvsazYfCNNUSkPqAVpSpNJ96U8trhNkfV0v9fk/edit#heading=h.tz3107poa9yf))
- Any broken app has long been broken in some legitimate scenarios (relying on non-standard behavior), this change just makes the breakage more consistent and predictable ([Maximizing ecosystem benefit](https://docs.google.com/document/d/1RC-pBBvsazYfCNNUSkPqAVpSpNJ96U8trhNkfV0v9fk/edit#heading=h.qgj6catobk6m))
- It's too late for the latest Chrome and Safari releases to avoid the bulk of the harm.  Reverting at this point at best reduces the impact for Chrome users ~6 weeks from now but likely not for Safari users (so, eg. iPhone users who have no choice but to use the Safari engine for web browsing would still be broken with no option but to switch device).

I'm sorry for pain this has caused the few impacted applications.  No amount of breakage ever seems acceptable when it's your app that's impacted.  But please understand that our primary concern here is to make the best possible trade-off for the health of the web overall, and at this point it seems likely that keeping this change will result in less total developer and user pain in the long-run than changing course would now.

If it would help, we could explore adding a temporary option in chrome://flags to restore the old behavior.  But that's a chromium implementation discussion, not a spec discussion, so please register any interest in that [on the chromium bug](https://bugs.chromium.org/p/chromium/issues/detail?id=749086).

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