[strcut_xml][fix] fix parse Chinese text in xml

[qicosmos]

Why

Close #520

What is changing

Example

struct some_object {
  int id;
  std::string name;
};

int main() {
  std::string xml_str = R"(<some_object><id>20</id><name>小强</name></some_object>)";
  some_object obj2;
  iguana::from_xml(obj2, xml_str);
  assert(obj2.id == 20);
  assert(obj2.name == "小强");
}