Shutdown socket writes before close

Calling shutdown(2) with SHUT_WR before close(2) ensures that a FIN
packet is sent before any RST packet. Doing so ensures that clients
and proxies get a clean shutdown signal when the server terminates the
connection while an upload is pending.

Fix #872
You mean add a lingering close to gunicorn thread @c-nichols raised at the nginx forum and linked over [here](https://github.com/benoitc/gunicorn/issues/872#issuecomment-54918855) in #872?