Fix local roles adapter lookup when no traversal happened beforehand.

This is a rare issue occured because of the context guessing happing in the dynamic role adapter lookup (DynamicRolesUtility.get_role_adapter).

This an alternative proposal to @Tschanzt 's PR #29 

I'm in favor of this solution because it is more robust and does return results, although the results might not be exactly correct, because the role data (title, permission) is the one effective for the Plone site and not for the actual traversed object.