Internal Server Error on getCurrentExternalInputsStatus
Heyho,

currently my TV does not want to deliver its external inputs to the homebridge plugin.

```
[BraviaTVOS] Error: Internal Server Error
    at exports.request (/homebridge/node_modules/homebridge-bravia-tvos/node_modules/@seydx/bravia/lib/request.js:80:21)
    at processTicksAndRejections (internal/process/task_queues.js:95:5)
    at ServiceProtocol.invoke (/homebridge/node_modules/homebridge-bravia-tvos/node_modules/@seydx/bravia/lib/service.js:89:12)
    at exports.fetchInputs (/homebridge/node_modules/homebridge-bravia-tvos/src/accessories/accessory.utils.js:39:22)
    at Setup (/homebridge/node_modules/homebridge-bravia-tvos/src/accessories/accessory.setup.js:145:26)
    at BraviaOSPlatform.didFinishLaunching (/homebridge/node_modules/homebridge-bravia-tvos/src/platform.js:38:5) {
  title: 'Invalid Response',
  code: 500,
  soap: {},
  url: 'http://192.168.178.24/sony/avContent'
}
```

I tried requesting the endpoint manually using curl:
```
curl -X "POST" "http://192.168.178.24/sony/avContent" \
     -H 'X-Auth-PSK: 1234' \
     -H 'Content-Type: text/plain; charset=utf-8' \
     -d $'{
    "method": "getCurrentExternalInputsStatus",
    "id": 105,
    "params": [],
    "version": "1.1"
}
'
```

The TV answers with
```{
  "error": [
    500,
    "Internal Server Error"
  ],
  "id": 105
}
```


I've seen your issues https://github.com/SeydX/homebridge-bravia-tvos/issues/75 where you say that this will be fixed in version 4. 
Do you know by any chance how you fixed it? I cannot find the corresponding commit in the log =/