respect current url's choice of trailing slash when we're descending to ''

In some cases, users are running web servers for which the inclusion or omission of a trailing slash in a url is significant. In other words, `http://foo.com/my/app` and `http://foo.com/my/app/` are different. Currently, using `$this->setBrowserUrl("http://foo.com/my/app")` results in Selenium trying to load `http://foo.com/my/app`, because of how URL::descend() works. This fixes that issue by checking whether we are descending to '' (as in the case of launching a session), and if so, respecting the value of the URL as it has been explicitly set.