use string values when traversing, instead of unicode

[erral]

In some scenarios, I found that the path is a unicode string, and when that happens the unrestrictedTraverse refuses to traverse the object so it can't change the workflow history.

This change uses safe_unicode to make the unicode -> utf8 conversion and have a sane string to call to unrestrictedTraverse

[lepri]

Or you can use str function to convert.

[calvinhp]

Any reason not to merge this?

[cdw9]

this same problem happens in the other blueprints as well, we should fix it throughout the whole package

[sunew]

Fixed for all blueprints in https://github.com/collective/collective.jsonmigrator/pull/26