Fix sudo/switchuser feature when used with SAML auth plugin

The SAMLAuthenticationPlugin }} is not setting the {{UserIdentificationInfo.credentialsChecked property to true in its {{handleRetrievIdentity }}method.
It causes the NuxeoAuthenticationFilter.getPrincipalCheckingAuth method to raise an exception when used during the user switching process.

PR available here : https://github.com/nuxeo/nuxeo-lts/pull/425