bugfix: Avoid the SAML2 IdP resilient to failures when getting DOMNodeList attribute values.

Due to recent changes in the SAML2 library, when an attribute has a value that contains XML, its contents are returned as a DOMNodeList instead of a string. This causes problems when running as a proxy, since the SAML2 IdP will obtain attributes in a format that cannot be cast to string. Regardless of the attribute encoding configured in the IdP for a remote SP, we should handle those cases gracefully, so that the IdP don't end up in an uncaught exception.