private Retrofit createRetrofit() {
    return new Retrofit.Builder()
            .baseUrl(BASE_URL)
            .addConverterFactory(GsonConverterFactory.create(buildGson()))
            .client(okHttpClient())
            .build();
}

private Gson buildGson() {
    Gson gson = new GsonBuilder()
            .registerTypeAdapter(Integer.class, new IntegerNullAdapter())
            .registerTypeAdapter(int.class, new IntegerNullAdapter())
            .create();

    return gson;
}

public class IntegerNullAdapter extends TypeAdapter<Integer> {
    @Override
    public Integer read(JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return 0;
        }
        String stringValue = reader.nextString();

        try{
            return Integer.valueOf(stringValue);
        }catch(NumberFormatException e){
            return null;
        }
    }
    @Override
    public void write(JsonWriter writer, Integer value) throws IOException {
        if (value == null) {
            writer.nullValue();
            return;
        }
        writer.value(value);
    }
}