Fix to prevent current_path from breaking when current_url contains spaces

The 'current_path' method on the session object, uses 'current_ur', which uses the javascript 'window.location.toString()' (with capybara-webkit at least) to determine the current url. That string may or may not encode special characters like spaces, it seems to depend on the browser. Eg, 'http://www.example.com/path with spaces/index.html'. But firefox doesn't encode spaces, and neither does webkit, and passing a string with spaces to URI.decode raises an error.

This patch makes the RackTest browser behave like firefox, and then introduces a test & fix for the problem.