Iterating all keys in the registry raises HTTPError: 500 Server Error: Internal Server Error

[CharString]

The bug discoverd in 14e7fa6 has been reported: plone/plone.restapi#1189
When it gets resolved the break in

affen/tests/test_utils.py

Line 39 in 35da413

break # datetime values still throw a 500 error

can be removed. And the tests should have full branch coverage then.