Add responseXML property to stubbed requests

Nothing much more to say than the title. Can be used like so:

```js
var parser = new DOMParser;
var mockDoc = "<?xml version='1.0' encoding='UTF-8'?><fake>This is some XML</fake>";
var mockXML = parser.parseFromString(mockDoc, "text/xml");

jasmine.Ajax.stubRequest("a/fake/api").andReturn({
    contentType: "text/xml",
    responseXML: mockXML
});
```