[autoembed][pastetext] Paste as plain text will upcast link into a widget

[Marek Lewandowski]

This one came to me while analysing #13415.

1. Open any sample with CKEditor.
2. Use "Paste as plain text" button from toolbar.
3. Insert following text https://twitter.com/codinghorror/status/610320718635466752.
4. Accept the dialog.

Expected result: Should insert plain text.

Current result: Link is upcasted to the embed widget.

It might be related to #13415, since some of proposition there might prevent user from pasting a plain link text into his document. Then paste as plain text would be his last hope to do it.