Url.parse/toString should support fragment

Hi,

Starting with Wicket 6.16, we have issues redirecting to an absolute
URL containing a fragment: the # is escaped by Wicket.

Changes in the commit
https://fisheye6.atlassian.com/changelog/wicket-git?cs=e1f29e90ff3a35e117bb3ddc5ee2aefd900c13c8
are causing the regression but as far as we can see, the commit is not
the problem: the issue is that Url.parse then toString encodes the
anchor.

Before this commit, as we were using an absolute Url, we were in the
case where the Url wasn't escaped.

We think the right fix would be to make org.apache.wicket.request.Url
anchor aware.

Pull request to come.