document.readyState should be interactive in DOMContentLoaded event handler

The code below shows "interactive" in real browser while in htmlunit it's "loading". 
```html
<html>
    <head>
        <title>
            test
        </title>
        <script>
document.addEventListener('DOMContentLoaded', function () {
    alert('readyState='+document.readyState);
});
        </script>
    </head>
    <body>
        test   
    </body>
</html>
```
