Adds json marshaling of the need_task_ids field to json_object

I though that I broke this with e561305e0224e073b3521ebd32332bae39bb222f, but it doesn't look to me like ShellMsg ever included need_task_ids in the message sent, which explains why I was still seeing taskIds returned from spouts and bolts.

Either way, this adds it back (only for emits).