XMLHttpRequest-class is missing response-attribute #388

I think that there should be response-attribute in the XMLHttpRequest-class as stated in the XMLHttpRequest-specification.

I stumbled on to this when debugging 3rd party Javascript library which didn't work with HtmlUnit and it relied on xhr.response-attribute when handing Ajax responses.

Is there any workaround for this issue, for example, could I extend HtmlUnit's XMLHttpRequest-class and use it instead?

Thanks for any assistance.