[http-server] Move connection keep-alive handling.

This moves the code from the server's response handling
code to the response's initialize method.
[http-server] Simplify response initialize method.

Remove the direction: keyword argument and validation as it isn't
needed.
[http-server] Just create <request> rather than make it configurable.
[http-server] Collapse some whitespace.
Merge pull request #75 from dylan-lang/cleanups

Cleanups