in PY3 rely on the content_length calculated by webob.Response.

motivation and simple example: there was still a problem with
the code so far - cf. the docs...

works fine:
  s.say_hello('Chris')
  'Hello, Chris'

still ok:
  s.say_hello('Français')
  'Hello, Français'

failed, when there was a second non-ascii char involved:
  s.say_hello('Hello, Société Foo')
  boom!

fixed now, should be covered by a unit test.