Read one object body after its GLib.Type.OBJECT type byte.
When object_type is set (homogeneous object arrays), skip read_gtype and decode the property stream for that class. When wire GLib.Object is anonymous, decode as expected_type when that type implements Serializable.
| object_type |
element class when already read from an array header |
| expected_type |
GObject property type for anonymous nested objects |