Python3 unicode error with AuthorizedClient

Great catch from asn that with python3 our AuthorizedClient construction embeds
extra b'', malforming descriptor output...

  https://trac.torproject.org/projects/tor/ticket/31823#comment:19

Fixing this is a general fashion such that AuthorizedClient both normalizes its
field and can be constructed with unsupplied values randomized (so, for
example, you can create a AuthorizedClient with only an iv).