MissingNode: fix bug in is{Value,Container,MissingNode}() contract

The Javadoc says that for any given node, only one such method may return true.

For MissingNode however, true was returned for both isMissingNode() and
isValueNode().
Merge pull request #144 from fge/missingnodefix

MissingNode: fix bug in is{Value,Container,MissingNode}() contract