fixed issue with newer JDK's being more strict on xml attribute ids

Newer JDK's issue an exception when trying to get the Assertion by their ID:

javax.xml.crypto.dsig.XMLSignatureException: javax.xml.crypto.URIReferenceException: com.sun.org.apache.xml.internal.security.utils.resolver.ResourceResolverException: Cannot resolve element with ID ...

https://forums.oracle.com/thread/1538547

This line registers the ID attribute for the Assertion document.