Add support for simple values to QJsonDocument

At this moment QJsonDocument can be created only from arrays and objects. For any other input result is null as sayed in docs. Before Qt 5.10 it was correct implemenation due QJson was based on old RFC-4627. RFC-4267 says that A JSON text is a serialized object or array. But in Qt 5.10 is used RFC-7159. In RFC-7159 JSON text has new definition A JSON text is a serialized value.

BTW current QJsonDocument should be changed to reflect this standard change.