Websocket server not sending PING when Targetframework set to .NET 5

### Describe the bug
I am using the sample code from document : https://docs.microsoft.com/en-us/aspnet/core/fundamentals/websockets?view=aspnetcore-5.0

When TargetFramework set to .NET core 3.1 It will work. But if change TargetFramework to .NET 5, the client will not receve any ping message.


### To Reproduce
Here is the source code of smaple projects:
[WebScoketTest.zip](https://github.com/dotnet/aspnetcore/files/6162079/WebScoketTest.zip)

1. open server project `AspnetCoreWebsocketServer1\AspnetCoreWebsocketServer1`, change TargetFramework from project settings to asp.net core 3.1. F5 to run.
2. open and run client project `WebsocketClientTest`, it will connect to server and log ping message every 3 seconds.
3. stop the two projects.
4. change targetframework of server project to .NET 5.
5. start server project.
6. sthart client project. there will be no ping message received.

Code on server side:
```c#
public class Startup
{
// This method gets called by the runtime. Use this method to add services to the container.
// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
public void ConfigureServices(IServiceCollection services)
{
}

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
if (env.IsDevelopment())
{
app.UseDeveloperExceptionPage();
}

app.UseRouting();

var webSocketOptions = new WebSocketOptions()
{
KeepAliveInterval = TimeSpan.FromSeconds(3)
};
app.UseWebSockets(webSocketOptions);

app.Use(async (context, next) =>
{
if (context.Request.Path == "/ws")
{
if (context.WebSockets.IsWebSocketRequest)
{
using (WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync())
{
await Echo(context, webSocket);
}
}
else
{
context.Response.StatusCode = 400;
}
}
else
{
await next();
}

});
}

private async Task Echo(HttpContext context, WebSocket webSocket)
{
var buffer = new byte[1024 * 4];
WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment(buffer), CancellationToken.None);
while (!result.CloseStatus.HasValue)
{
await webSocket.SendAsync(new ArraySegment(buffer, 0, result.Count), result.MessageType, result.EndOfMessage, CancellationToken.None);

result = await webSocket.ReceiveAsync(new ArraySegment(buffer), CancellationToken.None);
}
await webSocket.CloseAsync(result.CloseStatus.Value, result.CloseStatusDescription, CancellationToken.None);
}
}
```
code on client side:
```c#
var ws = new WebSocket("ws://127.0.0.1:5000/ws");
ws.SetCookie(new Cookie("token", "demo"));
ws.SetCookie(new Cookie("device", "AmdDev"));
ws.EmitOnPing = true;

// add reference to avoid auto dispose
_sockets.Add(ws);

{
ws.OnMessage += (sender, e) =>
{
Console.WriteLine($"OnMessage, IsPing:{e.IsPing} IsText:{e.IsText} data:{e.Data}");
};

ws.OnClose += (sender, eventArgs) =>
{
Console.WriteLine(
$"OnClose... Code:{eventArgs.Code} Reason:{eventArgs.Reason} WasClean:{eventArgs.WasClean}");
};

ws.OnError += (sender, eventArgs) =>
{
Console.WriteLine($"OnError!! Message:{eventArgs.Message} Exception:{eventArgs.Exception?.Message}");
};

ws.OnOpen += (sender, eventArgs) =>
{
Console.WriteLine($"OnOpen ");
ws.Send($"client onOpen {index}");
};

ws.Connect();
}

```

### Exceptions (if any)

### Further technical details
- ASP.NET Core version : 3.1 and 5
- Include the output of `dotnet --info`
- The IDE (VS / VS Code/ VS4Mac) you're running on, and its version: VisualStudio Community 2019 16.9.2

output of `dotnet --info`
```
C:\Users\cuili>dotnet --info
.NET SDK (反映任何 global.json):
Version: 5.0.201
Commit: a09bd5c86c

运行时环境:
OS Name: Windows
OS Version: 10.0.19042
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\5.0.201\

Host (useful for support):
Version: 5.0.4
Commit: f27d337295

.NET SDKs installed:
3.1.103 [C:\Program Files\dotnet\sdk]
3.1.201 [C:\Program Files\dotnet\sdk]
5.0.100-rc.1.20452.10 [C:\Program Files\dotnet\sdk]
5.0.200-preview.21079.7 [C:\Program Files\dotnet\sdk]
5.0.201 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
Microsoft.AspNetCore.All 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.26 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.26 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.0-rc.1.20451.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 2.1.23 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.26 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.0-rc.1.20451.14 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.3 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.9 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 3.1.13 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.0-rc.1.20452.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.2 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
Microsoft.WindowsDesktop.App 5.0.4 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

To install additional .NET runtimes or SDKs:
https://aka.ms/dotnet-download
```

---
#### Document Details

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* ID: a3b0cac1-adc7-434c-d867-6e28e39bdae7
* Version Independent ID: 762efeed-010e-f422-03d0-22d6681b2851
* Content: [WebSockets support in ASP.NET Core](https://learn.microsoft.com/en-us/aspnet/core/fundamentals/websockets?view=aspnetcore-5.0)
* Content Source: [aspnetcore/fundamentals/websockets.md](https://github.com/dotnet/AspNetCore.Docs/blob/main/aspnetcore/fundamentals/websockets.md)
* Product: **aspnet-core**
* Technology: **aspnetcore-fundamentals**
* GitHub Login: @Rick-Anderson
* Microsoft Alias: **riande**