Support null values for JsonObject
 - putArray, putElement, putBinary, and putValue now support null values.

Fixes bug https://bugs.eclipse.org/bugs/show_bug.cgi?id=426816

Signed-off-by: nscavell <nick.scavelli@gmail.com>
Merge pull request #783 from nscavell/npe-jsonobject

Support null values for JsonObject