"result" should always be an object (so that we can add new fields),
so make that implicit in json_stream_success.
This makes our primitives well-formed: we previously used NULL as our
fieldname when calling the first json_object_start, which is a hack
since we're actually in an object and the fieldname is 'result' (which
was already written by json_object_start).
There were only two cases which didn't do this:
1. dev-memdump returned an array. No API guarantees on this.
2. shutdown returned a string.
I temporarily made shutdown return an empty object, which shouldn't
break anything, but I want to fix that later anyway.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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