UndoManager.locked volates strict mode in undo plugin

[Marek Lewandowski]

While working on #11739 I found out that at least these two linex violates strict mode (Safari6):

• ​place1
• ​place2

The reason for that is that (at least sometimes) ​undoManager.locked becomes a number. And if you attempt to add new property directly to a Number object it will cause a strict violation.

Steps to repro:

1. ensure that your ​plugins/undo/plugin.js has a use strict statement
2. run ​plugins/undo/change test

Expected result:
Tests should pass.

Current result:
Tests test undo and test redo will fail.

additional info:

1. I've pushed temp workaround for this issue to ​t/11739_safari_fix branch.