Use `sent` instead of `response_length` in access log formatter

Currently, `resp.response_length` is used for the b atom of the access log formatter. If the application does not set a content-length header, `resp.response_length` is `None`, resulting in the b atom being set to the fallback "-".

`resp.sent` is a more reliable way to determine the number of bytes actually included in the response body, which seems to more closely match the description of %b at http://httpd.apache.org/docs/2.0/logs.html#combined