Adding `types` properties into mocked DataTransfer

Hey there!

We've been using this addon quite intensively lately, thanks for the hard work! In order to be able to check the validity of the drag event, we ended up doing this:

import DraggableObjectTarget from 'ember-drag-drop/components/draggable-object-target';

export default class CustomDraggableObjectTarget extends DraggableObjectTarget {
  validateDragEvent(event) {
    return event.dataTransfer.types.length === 1 && event.dataTransfer.types.includes('text/plain');
  }
}

But the problem introduced is that we can no longer use the drag test-helper. This PR aims at adding a types property in the mocked DataTransfer :)

Let me know if you see anything I may rework!

Thanks!

mharris717 / ember-drag-drop

Adding `types` properties into mocked DataTransfer #191