Fix issue where no parsererror is returned on invalid XML

[m90]

This fixes #54 but would introduce the situation where using slightly broken DOMParsers like xmldom would make this test:

nise/lib/fake-xhr/index.test.js

Lines 1700 to 1708 in 929b9fe

	it("is null when the response body contains invalid XML", function () {
	this.xhr.open("GET", "/");
	this.xhr.send();
	this.xhr.respond(200, { "Content-Type": "application/xml" },
	"!!!<?xml version=\"1.0\" encoding=\"UTF-8\"?><broken>");
	assert.isNull(this.xhr.responseXML);
	});

fail as it would return a document instead of null.

[fatso83]

Thank you!

[senaev]

there is an error
#57