2.12.0: Reduced filter boilerplate

• Changed filters API so that any web.Filter implementation can use
  yield from $invocation->proceed(...) without having to test whether
  handlers return a generator first, e.g. via is_iterable(). Filters
  using return $invocation->proceed(...) continue to work. Keep in mind
  they return before asynchronous handlers have completely executed!
  (@thekid)