It's OK if the socket is closed by the client side.

This PR addresses issue #522.

The try-except around `sock.shutdown(...)` prevents worker exceptions like these:

```
2013-04-29T22:30:46.720Z ERROR N=gunicorn.error P=69755 Socket error processing request.
Traceback (most recent call last):
  File ".../src/gunicorn/gunicorn/workers/async.py", line 45, in handle
    self.handle_request(listener, req, client, addr)
  File ".../src/gunicorn/gunicorn/workers/ggevent.py", line 119, in handle_request
    super(GeventWorker, self).handle_request(*args)
  File ".../src/gunicorn/gunicorn/workers/async.py", line 111, in handle_request
    sock.shutdown(socket.SHUT_RDWR)
  File ".../src/witness/lib/python2.7/site-packages/gevent-1.0rc2-py2.7-macosx-10.8-x86_64.egg/gevent/socket.py", line 517, in shutdown
    self._sock.shutdown(how)
error: [Errno 57] Socket is not connected
```