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

Previously, resp.response_length is used for the b atom of the access
log formatter. If the application does not set a content-length header,
response_length is left as `None`, resulting in b being set to the
fallback "-". resp.sent is a more reliable way to determine the number
of bytes in the response body.
Pass a 'sent' parameter to Mock().