[refactor] Fix wrong use of json marshaller in httpAckNacker
[refactor] Fix wrong use of json marshaller in httpAckNacker

mgranberry/ttn,LoRaWanSoFa/ttn,LoRaWanSoFa/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,mgranberry/ttn,TheThingsNetwork/ttn,TheThingsNetwork/ttn,jvanmalder/ttn,mgranberry/ttn,jvanmalder/ttn