unicode error fix in transaction_note

Proposed bugfix for https://dev.plone.org/ticket/12782

I default to us-ascii for now. I wanted to use the site encoding, but since I don't have a context, it's not straightforward to get the site encoding.

us-ascii is used in the utils.py; so that's why I defaulted on that encoding. Is this a sane default, or should I take something else (utf-8 has been suggested).