QML drag source is wrong if dragType == Drag.Automatic

Please try the attached test case. When you start dragging one rectangle, the message appearing on the console should say:

qml: Drag source is QQuickRectangle(0x...)

but it says:

qml: Drag source is QQuickDragAttached(0x...)

This does not happen if the Drag.dragType: Drag.Automatic line is removed: internal drags work fine.
The problem only affects external DnD operation, and I think it's due to the way that the QDrag object is constructed in src/quick/items/qquickdrag.cpp. I'll try to prepare a patch.