PageModelSerializerModifier should not return the specialized PageModelSerializer for every object

Currently it contains this"

    @Override
    public JsonSerializer<?> modifySerializer(SerializationConfig config, BeanDescription beanDesc,
                                              JsonSerializer<?> serializer) {
        return new PageModelSerializer((JsonSerializer<Object>) serializer, jsonPointerFactory, metadataDecorators);
    }

However, when serializing a String object for example, there is no point in returning the PageModelSerializer.

We only need to return the PageModelSerializer for

1. Objects that implement PageModelEntity (see HSTTWO-4750)
2. The object AggregatedPageModel.RootReference

All the other objects can just be serialized using the default object mapper serialized and do not need our enhanced PageModelSerializer