yahns 1.4.0

Minor internal cleanups and bugfixes.

* http_client: clear some internal ivars on rack.hijack
* README: minor updates
* extras/autoindex: do not link beyond top-level
* extras/autoindex: fix misnamed variable
* extras/autoindex: integrate with TryGzipStatic
* queue: remove TODO comment for kqueue
* switch to require_relative for internal requires
* README: add OpenSSL exception
* stream_input: favor str.clear instead of str.replace('')
* http_response: skip Date header case-insensitively