Duplicated Content-Length headers on pass

[Carlos Valiente]

> Feedback on whether this bug poses a real-world problem for people or
> not would be most welcome.

Thanks for the workaround and the patch, Tollef. Chromium (I'm using
https://launchpad.net/~chromium-daily/+archive/ppa) refuses to render
pages served with a duplicate ``Content-Length`` header, explicitly
mentioning it on its error message.

Cheers,

Carlos