fix for undoManager isUndoRegistrationEnabled

UndoRegistration should be enabled by default.
https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSUndoManager_Class/Reference/Reference.html#//apple_ref/occ/instm/NSUndoManager/isUndoRegistrationEnabled

Currently it is not because the method only test for "_disabledCount == 0" which does not account for _disableCount being null which it is at start.

Here is a simple test:
```
console.log("UndoRegistration should be enabled by default, this should return true: " + [[theWindow undoManager]  isUndoRegistrationEnabled]);
[[theWindow undoManager]  disableUndoRegistration];
console.log("UndoRegistration is disabled, this should return false: " + [[theWindow undoManager]  isUndoRegistrationEnabled]);
[[theWindow undoManager]  enableUndoRegistration];
console.log("UndoRegistration is enabled, this should return true: " + [[theWindow undoManager]  isUndoRegistrationEnabled]);
```