OAK-781: Clarify / fix effects of MISSING_NODE as base state of NodeBuilder
remove failing assertion that does not hold anymore since the introduction of the exists semantics on node states. Extend test case

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1468923 13f79535-47bb-0310-9956-ffa450edef68