[MAJOR] http: rework response Connection header handling

[MAJOR] http: rework response Connection header handling

This one is the next step of previous patch. It correctly computes
the response mode and the Connection flag transformations depending
on the request mode and version, and the response version and headers.

We're now also able to add "Connection: keep-alive", and to convert
server's close during a keep-alive connection to a server-close
connection.