[P1] Support passing subclasses

[DMwangnima]

codec-dubbo needs to support passing subclasses. Example: api.thrift:

list<java.Date> EchoJavaDateList(1: list<java.Date> req)(hessian.argsType="java.util.List")

dubbo-java:

    public List<Date> EchoJavaDateList(List<Date> req) throws Exception {
        List<Date> returnDates = new ArrayList<>();
        for (Date date : req) {
            returnDates.add(new java.sql.Timestamp(date.getTime()));
        }
        return returnDates;
    }

As for dubbo-java <-> dubbo-java, returning subclass works. But for dubbo-go-hessian2, it just decode the returning contents as map[string]interface{}.