fix WebSocket cannot send/receive more than 4096 bytes of data

I have setup a temporary websocket server for yr testing.
And, the write buffer size reduced to 2048. I re-tested 4096 is okay in intranet, but failed on internet. 2048 is the safe size tested.

here is the test case in LUA:

  ws = WebSocket:create("ws://118.26.229.45:8000")
  
  ws:registerScriptHandler(function(strData)
    print('Websocket Open: ', strData)
    local data = ''
    for i=1, 2560 do
      data = data .. '0123456789abcdef'
    end
    -- send out 40k string
    ws:sendString(data)
  end,cc.WEBSOCKET_OPEN)
  
  ws:registerScriptHandler(function(strData)
    print('Websocket Message: ', #strData, strData)    
  end,cc.WEBSOCKET_MESSAGE)