Fake XHR XML document parser fails to parse.

[bafolts]

It looks like there is an error attempting to determine what a parse error may look like.

https://github.com/sinonjs/nise/blob/master/lib/fake-xhr/index.js#L345

I have a situation running with a DOMParser stub in node where the document INVALID does not return any elements with a parseerror tag name. In this situation my XML is failing to parse even though my XML is valid. The check for getElementsByTagName("parsererror") should verify that it got a result prior to checking the item at index 0. In my situation I have valid XML but no parsererror for the "INVALID" document.