Default action of Enter and ShiftEnter should be always prevented

[pomek]

Suggested merge commit message (convention)

Fix: Default action of Enter event should be always prevented. Closes ckeditor/ckeditor5#1120.

[pomek]

I tried to write an integration test in image package but it always passed...

I wanted to fire the enter event on view.document. Was it a correct way?

[coveralls]

Coverage remained the same at 100.0% when pulling 4ae6a99 on t/ckeditor5/1120 into facc882 on master.

[Reinmar]

I wanted to fire the enter event on view.document. Was it a correct way?

Yes.

[pomek]

The tests are here - ckeditor/ckeditor5-image#218

But as I said, I am not sure about them.

[Reinmar]

You wrote that they always passed. That indeed makes them invalid. I'll look into this.

[Reinmar]

OK, I realised now that you're talking about the integration tests. They indeed check a different thing. I'll comment that in that other PR.

[Reinmar]

I merged this PR because itself it's correct.