Implement JValue::duplicate()

  The JValue should be regarded as a shared_ptr, copies of which all
  reference the same object. If a JSON value needs be cloned, it must be
  stated explicitly.

[GF-4132] libpbnjson: original value is changed when copied

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: I0ec74271d93a98b9eb83381adf38c9f7638a8d22