[IE8] Coping and pasting some widgets results in invisible (empty) widgets

[Piotrek Koszuliński]

1. See ​http://tests.ckeditor.dev:1030/tests/plugins/embed/manual/embed
2. Try to copy&paste any of the widgets.
3. Empty widget is pasted.

Some details:

• This happens only with some kinds of widgets. It cannot be reproduced with image2, but can be with code snippets.
• This isn't a regression. We didn't notice this (and no one reported this - I guess no one uses widgets with IE8) because code snippets aren't supported on IE8.