[wasm] WasmAppHost: fix new analyzer warning

`/__w/1/s/src/mono/wasm/host/WebServerStartup.cs(94,17): error ASP0019: Use IHeaderDictionary.Append or the indexer to append or set headers. IDictionary.Add will throw an ArgumentException when attempting to add a duplicate key. [/__w/1/s/src/mono/wasm/host/WasmAppHost.csproj]
`

Author: radical

src/mono/wasm/host/WebServerStartup.cs

@@ -13,6 +13,7 @@
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Routing;
 using Microsoft.AspNetCore.StaticFiles;
 using Microsoft.Extensions.FileProviders;
@@ -91,8 +92,8 @@ public void Configure(IApplicationBuilder app,
         {
             app.Use((context, next) =>
             {
-                context.Response.Headers.Add("Cross-Origin-Embedder-Policy", "require-corp");
-                context.Response.Headers.Add("Cross-Origin-Opener-Policy", "same-origin");
+                context.Response.Headers.Append("Cross-Origin-Embedder-Policy", "require-corp");
+                context.Response.Headers.Append("Cross-Origin-Opener-Policy", "same-origin");
                 return next();
             });
         }