WebApplication doesn't recognize if an incoming request is multipart.

Thanks to the mail at http://apache-wicket.1842946.n4.nabble.com/Read-POST-based-request-from-external-site-td4651269.html we have spotted a problem with method newWebRequest of class WebApplication.
It seems that this method doesn't test if the original request is multipart and doing so post parameters go lost.
We should create a MultipartServletWebRequestImpl when such a type of request is being served. I attach a possible patch but I'm not 100% about two things:

• which is the best way to determinate if a HttpServletRequest is multipart?
• in order to build a MultipartServletWebRequestImpl we need to provide a string identifier for the upload. How can we generate it (in my patch it's a constant value)?