XML documents do not use the "determine the encoding" algorithm. As far
as I can tell, it is unspecified how they should determine the encoding
instead. We now check the BOM, `Content-Type` header and prescan for an
xml encoding declaration (but don't inherit encodings from iframes or
attempt to determine the encoding from heuristics).
Testing: New tests start to pass
Part of https://github.com/servo/servo/issues/6414
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>