UndoManager clears when entering text and there are deltas in the redo stack

[bjnsn]

To reproduce do the following waiting a second between each step to allow the undo manager to handle them separately:

type x
type y
-undo-
type z

An error is caught in the UndoManager record method which causes the undo & redo stacks to be cleared. This is because the 'oldDelta' is out of date when an undo is fired.

[jhchen]

This should be fixed with the PR merge