Fix WP_User constructor in shibboleth_authenticate_user

According to the WordPress documentation, the WP_User constructor takes
`User's ID, a WP_User object, or a user object from the DB.` as its first parameter.

As I understand it, that means the plugin in this revision is trying to pass a username as this parameter, which will therefore never succeed to get the appropriate user data.

Passing `0` as the first parameter is the same as the default constructor, and fixes this error in my testing.
source: http://developer.wordpress.org/reference/classes/wp_user/__construct/