Add an exception to deal with Set-Cookie header

[yohanboniface]

According to https://tools.ietf.org/html/rfc7230#page-23,
Set-Cookie is the only response header field that can appear
multiple times.
So let's deal with that as an exception to keep normal perf
in normal case.

[yohanboniface]

This should actually change when we'll have a proper cookie management.

[davidbgk]

In the process of trying to add more tests, I wondered if some response headers are required. It appears that none is really required but a few are advised. Don't know where we want to go and that's probably not relevant in that PR, that's more to discuss.

Somehow related: an empty response is currently b'HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n' maybe we can remove the Content-Length: 0 in that case?

[yohanboniface]

Don't know where we want to go and that's probably not relevant in that PR, that's more to discuss.

#26

Somehow related: an empty response is currently b'HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n' maybe we can remove the Content-Length: 0 in that case?

I'm tempted to say yes, but it may be that some clients user Content-Length: 0 as a way to know there is no body to come (neither a stream I mean) ? Anyway, let's discuss this in #26.