fixed websocket subprotocol negotiation

Frameworks/MQTTnet.AspnetCore/ApplicationBuilderExtensions.cs

@@ -2,6 +2,7 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.Extensions.DependencyInjection;
 using MQTTnet.Core.Server;
+using System.Linq;
 
 namespace MQTTnet.AspNetCore
 {
@@ -14,8 +15,21 @@ namespace MQTTnet.AspNetCore
             {
                 if (context.Request.Path == path && context.WebSockets.IsWebSocketRequest)
                 {
+                    string subprotocol = null;
+
+                    if (context.Request.Headers.TryGetValue("Sec-WebSocket-Protocol", out var requestedSubProtocolValues))
+                    {
+                        if (requestedSubProtocolValues.Count > 0)
+                        {
+                            if (requestedSubProtocolValues.Any(v => v.ToLower() == "mqtt"))
+                            {
+                                subprotocol = "mqtt";
+                            }
+                        }                            
+                    }
+
                     var adapter = app.ApplicationServices.GetRequiredService<MqttWebSocketServerAdapter>();
-                    using (var webSocket = await context.WebSockets.AcceptWebSocketAsync("mqtt"))
+                    using (var webSocket = await context.WebSockets.AcceptWebSocketAsync(subprotocol))
                     {
                         await adapter.AcceptWebSocketAsync(webSocket);
                     }

Tests/MQTTnet.TestApp.NetCore/ClientTest.cs

@@ -21,10 +21,10 @@ namespace MQTTnet.TestApp.NetCore
                     {
                         Server = "localhost"
                     },
-                    ////ChannelOptions = new MqttClientWebSocketOptions
+                    //ChannelOptions = new MqttClientWebSocketOptions
-                    ////{
+                    //{
-                    ////    Uri = "localhost"
+                    //    Uri = "ws://localhost:59690/mqtt"
-                    ////}
+                    //}
                 };
 
                 var factory = new MqttFactory();