无法在asp.net核心中为SPA文件设置HTTP头

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using System.Threading.Tasks;

public class SpaHeaderMiddleware
{
    private readonly RequestDelegate _next;

    public SpaHeaderMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        // 设置HTTP头
        context.Response.Headers.Add("Custom-Header", "Value");

        await _next(context);
    }
}

public static class SpaHeaderMiddlewareExtensions
{
    public static IApplicationBuilder UseSpaHeader(this IApplicationBuilder builder)
    {
        return builder.UseMiddleware<SpaHeaderMiddleware>();
    }
}

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
    // ...

    app.UseSpaHeader();

    // ...
}