fix: avoid setting User-Agent header in .NET Framework (#25)

* avoid setting User-Agent header in .NET Framework

* exclude net462 example project

* revert wrong change

* remove NET462 Console App example

* add reference

Author: deleteLater

src/FeatBit.ServerSdk/Transport/WebSocketTransport.cs

@@ -91,7 +91,16 @@ public async Task StartAsync(
         private static async Task<WebSocket> DefaultWebSocketFactory(Uri uri, CancellationToken cancellationToken)
         {
             var webSocket = new ClientWebSocket();
+
+            // reference: https://github.com/dotnet/aspnetcore/blob/main/src/SignalR/clients/csharp/Http.Connections.Client/src/Internal/WebSocketsTransport.cs#L98-L105
+            // Full Framework will throw when trying to set the User-Agent header
+            // So avoid setting it in netstandard2.0 and only set it in netstandard2.1 and higher
+#if !NETSTANDARD2_0 && !NETFRAMEWORK
             webSocket.Options.SetRequestHeader("User-Agent", HttpConstants.UserAgent);
+#else
+            // Set an alternative user agent header on Full framework
+            webSocket.Options.SetRequestHeader("X-FeatBit-User-Agent", HttpConstants.UserAgent);
+#endif
 
             try
             {