Flaw in QJsonObject makes inserting values from the same object fail

Copying values in QJsonObjects sometimes doesn't work:

    QJsonObject obj;
    obj["value"] = "TEST";
    obj["prop2"] = obj["value"];
    qDebug() << obj;

Output:

QJsonObject({"prop2":null,"value":"TEST"})

The problem is that QJsonObject keeps its items sorted lexicographically, so inserting "prop2" makes the QJsonValueRef of obj["value"] point to the newly inserted item.

Tested with both 5.13.2 (original binary QJsonDocument implementation) and 5.15.0 beta 4 (QCborContainerPrivate implementation).