update JSON-RPC codec to observe specification

[benjaminbollen]

rpc: update JSON-RPC codec to follow the specification wrt Response object;
specifically the result and error member of the Response object are mutually exclusive per http://www.jsonrpc.org/specification#response_object

fixes #464

[benjaminbollen]

for erisdb.GetAccounts I now get on an empty chain:

$ curl -H "Content-Type: application/json" -X POST --data '{"id":"0","method":"erisdb.getAccounts","params":{"filters":[]},"jsonrpc":"2.0"}' http://localhost:1337/rpc | jq .
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   515  100   435  100    80   114k  21557 --:--:-- --:--:-- --:--:--  141k
{
  "result": {
    "accounts": [
      {
        "address": "0000000000000000000000000000000000000000",
        "pub_key": null,
        "sequence": 0,
        "balance": 1337,
        "code": "",
        "storage_root": "",
        "permissions": {
          "base": {
            "perms": 2302,
            "set": 16383
          },
          "roles": []
        }
      },
      {
        "address": "DDE80114E6BFED2EC6A823B30FC1F47B4B9C72B3",
        "pub_key": null,
        "sequence": 0,
        "balance": 99999999999999,
        "code": "",
        "storage_root": "",
        "permissions": {
          "base": {
            "perms": 16383,
            "set": 16383
          },
          "roles": []
        }
      }
    ]
  },
  "id": "0",
  "jsonrpc": "2.0"
}

and for an error:

curl -H "Content-Type: application/json" -X POST --data '{"id":"0","method":"erisdb.getAccounts","params":{"filters":[THIS-IS-AN-INVALID-FILTER]},"jsonrpc":"2.0"}' http://localhost:1337/rpc | jq .
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   244  100   139  100   105  33582  25368 --:--:-- --:--:-- --:--:-- 46333
{
  "error": {
    "code": -32700,
    "message": "Failed to parse request: invalid character 'T' looking for beginning of value"
  },
  "id": "",
  "jsonrpc": "2.0"
}

[silasdavis]

Happy to merge, unless we want to switch to marker interface. Just has the advantage of adding intentionality to the struct definer rather than the caller who is passing that struct.

[silasdavis]

How about making this an interface with a marker method func AssertIsRPCResponse()?