Without really making big the in the API, one way of providing a position sensitive methods to enable more organized headers is to have. Having an index based api probably needs `more` discussion and thought!

1. add_header_before('existing-header-name', 'new-name', value)
2. add_header_after('existing-header-name', 'new-name', value)

The `existing-header-name` is again taken by first-found.

If put all routing, forwarded headers, one can always use `add_header_before('From'...)`.