Apply serialize nulls for specific classes only

I want a Gson setup in which nulls are not serialized by default. I want nulls to be serialized for certain class only. This is how I tried to solve it, but it does not work as expected.

This is how a gson instance is created, used for general serialization/deserialization for all API requests app needs to make:

```
@Provides
Gson provideGson() {
GsonBuilder builder = new GsonBuilder()
.registerTypeAdapter(SubmitBody.class, new SubmitBodySerializer();
return builder.create();
}
```

I want to make `SubmitBody` class an exception for serialization, I want null fields from this class to be serialized as Json nulls, unlike rest of the classes where such fields will be ignored. That's why I registered a custom serializer for this class that looks like this:

```
public class SubmitBodySerializer implements JsonSerializer {

public SubmitBodySerializer(Gson gson) {
this.gson = new GsonBuilder().serializeNulls().create();
}

@Override
public JsonElement serialize(SubmitBody src, Type typeOfSrc, JsonSerializationContext context) {
return gson.toJsonTree(src);
}
}
```

Within this class I use a separate Gson instance which serializes nulls. I can see that serialize method returns correct JsonElement with serialized nulls, however, when I check the Json body sent to the API, nulls are nowhere to be found, as if they are ignored.