[struct_json][fix]fix json

[qicosmos]

Why

fix a bug in from_json.

What is changing

Example

struct person1 {
  std::shared_ptr<std::string> name;
  std::unique_ptr<int64_t> age;
};
REFLECTION(person1, name, age);

void use_smart_pointer() {
  person1 p{std::make_shared<std::string>("tom"),
            std::make_unique<int64_t>(42)};
  std::string str;
  iguana::to_json(p, str);

  person1 p1;
  iguana::from_json(p1, str); 

  assert(*p1.name == "tom");
  assert(*p1.age == 42);
}