The WSGI specification requires that all middlewares and gateways
respect the `close` callback of the iterable returned by the application.
Because it is useful to add another close action to a returned iterable
and adding a custom iterable is a boring task this class can be used for
that::
return ClosingIterator(app(environ, start_response), [cleanup_session,
cleanup_locals])
If there is just one close function it can be passed instead of the list.
A closing iterator is not needed if the application uses response objects
and finishes the processing if the response is started::
try:
return response(environ, start_response)
finally:
cleanup_session()
cleanup_locals()