dragStart callback only fires when canDrag returns true

[tedbarth]

When canDrag returns false when initiating drag interaction as user, the onDragStart callback is not called.

When holding the drag interaction (mouse button still pressed) the canDrag callback is called about every second and, when dragging the item to a position where the canDrag callback returns true, onDrag is called and when dropping it accepts the drag action, resulting in a onDrop callback call. This works like expected, though.

It is just that onDragStart is never called when canDrag returns false when initiating drag interaction.

Tested with 1.7.0, 1.7.1, 1.7.2, 1.7.3