snr field is missing when value is 0

[tftelkamp]

In the MQTT uplink message, the "snr" field is missing when a gateway sends a 0 value. For example:

{"rxInfo":{"mac":"0000024b080000af","time":"2017-08-22T12:26:05.36343Z","timestamp":3997134452,"frequency":867100000,"channel":3,"rfChain":0,"crcStatus":1,"codeRate":"4/5","rssi":-118,"loRaSNR":0,"size":21,"dataRate":{"modulation":"LORA","spreadFactor":11,"bandwidth":125}},"phyPayload":"removed"}

becomes in TTN:

{"gtw_id":"eui-0000024b080000af","timestamp":3997134452,"time":"2017-08-22T12:26:05.36343Z","channel":3,"rssi":-118,"latitude":52.1234,"longitude":5.1234,"altitude":-31}]}