Pasting as plain text into pre blocks shouldn't replace pre

[Karen Ananiev]

Create pre block, copy text from any text editor and try to paste as plain text. Pre block will be removed and text will be pasted as paragraphs.

[Jakub Ś]

To reproduce:

1. Open attached files and copy contents
2. Clear editor contents and set formatting to "Formatted text"
3. Paste Notepad contents with Ctrl+V

Result: pre is removed in every browser

Problem can be reproduced from CKEditor 4.0 beta.

Problem could also be reproduced in 3.x (from CKE 3.2.1) in IE and Opera but from CKEditor 4.0 beta results were unified in all browsers thus I'm setting 4.0 beta as start version.