shibboleth: Fix connection when the URL do not have a path

When the url do not have a path (for example: "http://example.com" as
opposed to "http://example.com/"),  its path is not a prefix of the
root path of the cookie (usually '/')

By adding the dav path, we make sure the URL has a path.

This made a bug when the owncloud url was just a domain name and did not
have a path