From_json not importing all fields

`from_json` pack method does not import every field available in the json payload, only name and value

```from kpn_senml import *
pack = SenmlPack("telemetry")
pack.from_json('[{"n": "iot:sensor:water", "v": 144.11811140946955, "u": "l/s", "bn": "telemetry"}]')
print(pack.to_json())
[{"n": "iot:sensor:water", "v": 144.11811140946955, "bn": "telemetry"}]
```
