http: keep-alive should default with HTTP/1.1 server

As RFC 2616 says we should, assume that servers will provide a persistent
connection by default.

> A significant difference between HTTP/1.1 and earlier versions of
> HTTP is that persistent connections are the default behavior of any
> HTTP connection. That is, unless otherwise indicated, the client
> SHOULD assume that the server will maintain a persistent connection,
> even after error responses from the server.

> HTTP/1.1 applications that do not support persistent connections MUST
> include the "close" connection option in every message.

Fixes #2436.