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 successfullyParsed is true
TEST COMPLETE