Support "/" as virtual-root

When the virtual-root was a single "/", it would be normalized to NULL due
to removal of trailing slashes, which in turn would fool us to belive that
we shouldn't generate virtual urls.

This makes the "/" normalize to "", effectively allowing virtual urls like
http://example.com/projectname to be generated without specifying the
full domain name as the virtual root.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>