Servo isn't strict enough about what XMLHttpRequest response XML it will accept

[pshaughn]

WPT xhr/responsexml-basic, responsexml-media-type, and responsexml-non-well-formed reveal cases where an XMLHttpRequest shouldn't be producing an XML document but is:

• an empty body
• a body that's syntactically XML, but with a Content-Type header that says it's something else
• lightly malformed XML

By WPT expectations and other browser behavior, these shouldn't parse into an XML document when they're the response to an XHR.

[pshaughn]

https://xhr.spec.whatwg.org/#document-response step 5 says "the result of running the XML parser", with explanatory language that this should fail if not well-formed. Servo's XMLHttpRequest::document_response calls ServoParser::parse_xml_document here, which does not have any way of returning a failure result.