Properly check whether the draggable attribute is true.

Checking whether an element has the ```draggable``` attribute does not provide the correct behavior. The ```draggable``` attribute can be present with the value ```false```. This patch correctly handles such cases.

See [The Draggable Attribute](https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Drag_operations#draggableattribute)