Update howler.core.js

[LIU9293]

Sometimes, users want to change the playlist, so i unload the howler instance and create a new one, but when i unload the instance, the '_onend' event listener will trigger every time.

In my case, the next method will affect the playlist too, so it bother me a lot. I think when people unload the instance, they do not want affect anything, so why emit the 'end' event ?

If you have some better idea, please let me know, thank you.