unicode error on copy + delete

[Christophe Combelles]

- from an empty zope3 instance, just using the zmi
- Create an object (ex: a folder) with a unicode name
- Copy the object (don't paste)
- Delete the object

[Christophe Combelles (ccomb)]

This leads to an unusable user account. The unicode name remains in the principal's clipboard.
The container traverser tries to getattr(container, unicodename)

[Christophe Combelles (ccomb)]

Here is a patch with unittest + ftest + fix.