[xhr] Should XHR store and send HTTP header names in lower case? (#34)

The XHR spec defers HTTP header handling to the Fetch spec, and the header names are byte-lowercased in Fetch spec.
https://xhr.spec.whatwg.org/#the-setrequestheader()-method
https://fetch.spec.whatwg.org/#terminology-headers

However, all major browers don't normalize them at this point. I think this is due to compatibility issue around legacy servers that may handle the header names in case-sensitive manner.
OTOH, I found it's beneficial for Chrome in terms of cleaner code and better performance to normalize all request / response header names to lowercase. 

So, I want to confirm that the XHR is expected to use lowercase header names. Or is that actually expected to pass through the overridden request headers as-is?

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/34