[WFLY-202] : Include registered attributes with no model representation in read-resource results

Adding test case to prove that bug is already fixed (probably since 7.2) since commit b2910f4a1884e0aab0a8c69bed4707c029dda578 seems to fix it at
https://github.com/wildfly/wildfly-core/blob/master/controller/src/main/java/org/jboss/as/controller/operations/global/ReadAttributeHandler.java#L166

Jira: https://issues.jboss.org/browse/WFLY-202