Previously, we were using an encoded string, which in practice worked,
but with python3, WSGI server will encode as latin1 (because WSGI is
latin1) and already with Zope4 on python2 we have issues with testing,
as functional testing fake WSGI server only accepts ascii headers [1]

1: https://github.com/zopefoundation/Zope/blob/cddecf7e/src/Testing/ZopeTestCase/functional.py#L125-L126