[bug report] dropping on unaccepted item, still can be done if already hovered pass through accepted ones

[michaelyohanes]

• My issue title starts with [bug report]
• I have read the README, especially the 'classic blunders' section, and the dragula docs.
• I have searched through the ng2-dragula issues for related problems, including closed issues.
• I have browsed through the issues labeled "future reference" for problems that have been solved before or have a known workaround.
• I am using the latest version of ng2-dragula.

Describe the bug (required)

We've been using ng2-dragula (latest version), and sets some items to be 'droppable' using accept option. While setting the others as non accepted ones.

Upon dragging the draggable item, passing through the accepted one.
We found out that dropping it into unaccepted item still can be done eventhough the target still detects as the previously hovered accept ones.

Please kindly assist

To Reproduce (required)

Steps to reproduce the behavior:

1. Set any item (not all of them) as accept on ng2-dragula options
2. Drag the draggable item hover pass the accept
3. Drop the draggable item into the item that not listed within the accept
4. It will still trigger the drop, and will still gave `target result (despite the target still remains to be the hovered one).

Versions (required)

• @angular/core: 8.2.13
• ng2-dragula: 2.1.1

Browsers affected (optional)
Tested on Google Chrome Version 78.0.3904.97 (Official Build) (64-bit)