Fixed a bug where paste didn't create an undo step

To reproduce bug:
Open an instance of TinyMCE
Copy some text "foobar"
type some content "testing"
paste (using keyboard shortcut)
undo

Expected: "foobar" is removed
Actual Result: "testing" and "foobar" is removed.

I used robot.js to perform the testing, so it's now loaded on the paste.html test page.