Minor improvements, and URL calculation fix

In URL::absoluteUrl(), a duplication of the subdirectory name happens in the generated URL when Thelia is installed in a subdirectory, and when $path was calculated with Router::generate(), which returns an absolute URL, starting at web server root, not Thelia root.

For example, if Thelia is installed in /thelia2, we got something like /thelia2/my/path
As base URL also contains /thelia2 (e.g. http://some.server.com/thelia2), we end up with http://some.server.com/thelia2/thelia2/my/path, instead of http://some.server.com/thelia2/my/path