title: Not able to use websockets module with multiple websocket connection
body: <p>Below code works fine one connection . But failed when number of connection is 2 </p>
  
  <pre><code>import asyncio
  import websockets
  
  USERS = []
  async def differnt():
      await USERS[0].send("Your name is ")
      name=await USERS[0].recv()
      print (name)
      await USERS[0].send("Your name is "+name) 
  
      if len(USERS) ==2:
          await USERS[1].send("Your name is ")
          name=await USERS[1].recv()
          print (name)
          await USERS[1].send("Your name is "+name)
  
  
  async def counter(websocket, path):
      USERS.append(websocket)
      print (USERS)
      await differnt()
  
  start_server = websockets.serve(counter, "localhost", 6789)
  asyncio.get_event_loop().run_until_complete(start_server)
  asyncio.get_event_loop().run_forever()
  </code></pre>
  
  <p>As soon as second connection added , it wont work . shows</p>
  
  <pre><code>    await USERS[0].send("Your name is ")
    File "/usr/local/lib64/python3.6/site-packages/websockets/protocol.py", line 555, in send
      await self.ensure_open()
    File "/usr/local/lib64/python3.6/site-packages/websockets/protocol.py", line 803, in ensure_open
      raise self.connection_closed_exc()
  websockets.exceptions.ConnectionClosedOK: code = 1000 (OK), no reason
  </code></pre>