Switch to expect strict encoded signature value
Switch to expect strict encoded signature value

After https://github.com/benoist/xmldsig/pull/33, our test cases no
longer pass because we're expecting a `<ds:SignatureValue>` with
wrapping, but the xmldsig library no longer wraps them.

ausaccessfed/saml-service,ausaccessfed/saml-service,ausaccessfed/saml-service,ausaccessfed/saml-service