Poco::JSON::Object::set() should return reference to this to allow chaining.

One should be able to do this:

```c++
JSON::Object obj{Poco::JSON_PRESERVE_KEY_ORDER};
obj
    .set("protocol"s, "v2"s)
    .set("device"s, device)
    .set("at"s, "now"s)
    .set("data"s, JSON::Object()
        .set("temperature"s, temperature));
```

Same for Array::add() and Array::set().
