fix line 449 TypeError

Exception happened during processing of request from ('127.0.0.1',
34421)
Traceback (most recent call last):
File "/home/cj/project/fgfw-lite/goagent/proxy.py", line 2217, in
do_CONNECT
    self.do_CONNECT_AGENT()
File "/home/cj/project/fgfw-lite/goagent/proxy.py", line 2305, in
do_CONNECT_AGENT
    self.do_METHOD()
File "/home/cj/project/fgfw-lite/goagent/proxy.py", line 2019, in
do_METHOD
    self.do_METHOD_FWD()
File "/home/cj/project/fgfw-lite/goagent/proxy.py", line 2034, in
do_METHOD_FWD
    response = http_util.request(self.command, self.path, payload,
self.headers, crlf=common.GAE_CRLF)
File "/home/cj/project/fgfw-lite/goagent/proxy.py", line 1322, in
request
    sock.close()
File "/home/cj/project/fgfw-lite/goagent/proxy.py", line 449, in close
    socket.socket.close(self._sock)
TypeError: unbound method close() must be called with socket instance as first argument (got NoneType instance instead)