use jester with SSE (server side event) example

[mrhdias]

In @alexeypetrushin example with the asynchttpserver lib, I replaced the "for" 
loop code with a "while". So I can test if the client is closed. But when I 
close the browser, the server continues to send the response to the client. 
Does the 
[isclosed](https://nim-lang.org/docs/asyncnet.html#isClosed%2CAsyncSocket) 
function detect when the connection to the browser is closed?
    
    
    import httpcore, asyncdispatch, strformat, asynchttpserver, asyncnet
    
    proc cb(req: Request): Future[void] {.async.} =
      let headers = newHttpHeaders(@[
        ("Content-Type", "text/event-stream"),
        ("Cache-Control", "no-cache"),
        ("Connection", "keep-alive"),
        # ("Connection", "close"),
        ("Access-Control-Allow-Origin", "*"),
        ("Content-Length", "") # To prevent AsyncHttpServer from adding 
content-length
      ])
      await req.respond(Http200, "200", headers)
      
      # for i in 1..5:
      var i = 0
      while not req.client.isclosed():
        echo "I'm alive ", $i
        let msg = "data: " & $i & "\n\n"
        await req.respond(Http200, msg)
        await sleep_async(1000)
        inc(i)
      # req.client.close() # <= closing, otherwise it's going to be kept alive 
and
                         # EvenSource in Browser won't reconnect automatically
    
    var server = new_async_http_server()
    async_check server.serve(Port(5000), cb, "localhost")
    echo "started"
    run_forever()
    
    
    Run