schema 1.1 commit: improving computing typed value of assertion xpath2 context variable $value, during following scenario,
$value represents value for a simpleType which is derived by restriction from a list and itemType of that list is a union.

git-svn-id: https://svn.apache.org/repos/asf/xerces/java/branches/xml-schema-1.1-dev@1429869 13f79535-47bb-0310-9956-ffa450edef68