[whatwg/xhr] Sort headers as if ASCII-uppercased (#248)

[Sorting headers lexicographically](https://github.com/whatwg/xhr/issues/50) after ASCII-lowercasing them has the property that application-defined header that begin with underscores sort to the beginning of the return string of `getAllResponseHeaders`. This [breaks](https://bugzilla.mozilla.org/show_bug.cgi?id=1540688) when JS accidentally assumes that the header of interest cannot be the first one.

I suggest doing the sorting as if the the header names had been ASCII-uppercased, to avoid sorting headers starting with an underscore first.

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