[xhr] Document responses and responseXML should be null in workers (#51)

Workers shouldn't have access to `Document` objects, and WPT even tests for this in workers/semantics/xhr/001.html, however the spec doesn't seem to say anything about `response` (where `responseType=="document"`) and `responseXML` when being evaluated from a worker context.

Perhaps we should be early-returning null in both cases?

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/51