[BUG] Can't update "deviceContent" property of IotHub Configuration

# Context
I would like to be able to update the deviceContent property through the library and/or through the API

# Problem encounter
When I call the "Registry.updateConfiguration(...)" all properties seems update but the deviceContent property keep old values.

# Code to reproduce
```Javascript
const registry = Registry.fromConnectionString("")

const configuration = {
id: 'extensions-test',
priority: 2,
targetCondition: "a valid query",
content: {
deviceContent: {
'properties.desired.extensions': {
id: "a custom id",
list: []
}
},
moduleContent: { },
modulesContent: { }
},
labels: { },
metrics: {
queries: { },
results: { }
},
etag: ''
}
await registry.addConfiguration(configuration)

configuration.content.deviceContent['properties.desired.extensions'] = {
id: "a new custom id",
list: [{ name: "a name", id: "a valid id" }]
}
await registry.updateConfiguration(configuration, true)
```
After the code above, the device content property has the old value ({ id: "a custom id", files: [] }) instead of the new one.

I also try to call the API (api-version=2020-09-30) directly but I have the same result...