Upgrade JSON default attributes (Java side)

The JSON variant attribute 'default' should use a TTCN-3 value as its parameter, which can also be a structured value or a reference, as defined in the TTCN-3 standard CR 9768 (http://oldforge.etsi.org/mantis/view.php?id=7968).

This has already been implemented on the Java side.

However, the implementation of C side introduced some inconsistency that has to checked, see 086db506.