Gson will fail to serialize a class "Foo" if you create a custom serializer
as follows:

public static class FooTypeAdapter implements JsonSerializer<Foo> {
  public JsonElement serialize(Foo src, Type typeOfSrc,
JsonSerializationContext context) {
    return context.serialize(src, typeOfSrc);
  }
}

Basically, it detects this as a circular reference.  Instead, we should
allow this kind of custom serializer because a client may want to perform
the default serialization of an object and then add new fields into the
JsonElement tree.

For example:
public static class FooTypeAdapter implements JsonSerializer<Foo> {
  public JsonElement serialize(Foo src, Type typeOfSrc,
JsonSerializationContext context) {
    JsonElement element = context.serialize(src, typeOfSrc);
    JsonObject jsonObject = element.getAsJsonObject();
    jsonObject.add("someNewProperty", new JsonPrimitive(1L));
    return jsonObject;
  }
}