AJAX file upload fails in IE8 due to Wicket.Ajax.handleMultipart() sniffing wicket:body tags as an HTML body element when running Wicket in development mode

To reproduce:

1. Run provided quickstart project
2. Open in IE8.
3. Upload a file.

Expected behaviour:

File is uploaded, and some basic information about uploaded file is displayed.

Observed behaviour:

IllegalStateException is thrown.

The IllegalStateException is being thrown because the form is being submitted as a normal form. This is because the JS in handleMultipart that traverses up the DOM looking for the actual form is checking tagnames, and in IE8, the tagName of a <wicket:body> is body, so as soon as it hits a wicket:body, it returns false. I've provided a patch that checks for identity against document.body - an alternative is to check that the form.tagUrn is empty, as it would be for a normal HTML element.