Test identification of XML content type in XHR responses.

On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".

PASS foo,bar/baz+xml
PASS foo;bar/baz+xml
PASS foo:bar/baz+xml
PASS foo\bar/baz+xml
PASS foo"bar/baz+xml
PASS foo/bar/baz+xml
PASS foo?bar/baz+xml
PASS foo=bar/baz+xml
PASS successfullyParsed is true

TEST COMPLETE