[BUG #8613] rewrite test for qx.io.request.Xhr
[BUG #8613] introduce method and async property
[BUG #8613] introduce phase property (instead of member var)
[BUG #8613] introduce readyState property (instead of member var)
[BUG #8613] introduce responseText property (instead of member var)
[BUG #8613] introduce responseXML property (instead of member var)
[BUG #8613] introduce status and statusText property (instead of member var)