Fix account_lines and account_offers result (RIPD-682)

The account_lines and account_offers RPC command results are omitting info because we are reassigning to the Json result variable stomping the previous value.

@JoelKatz  @rec
The real question is whether we want to include what we already placed in result, which would be ledger_index, ledger_hash etc... I think the combined result would be correct as opposed to just this error. We can easily just return jv as before. David please chime in.