借助中间件（middleware）可以“篡改”请求上下文信息（这里是Request.Path）的能力解决了这个问题

中间件的实现代码如下：

public class LowercaseUrlMiddleware
{
    private readonly RequestDelegate _next;
    private readonly IEnumerable<string> _wwwrootDirs;

    public LowercaseUrlMiddleware(RequestDelegate next, IEnumerable<string> wwwrootDirs)
    {
        _wwwrootDirs = wwwrootDirs;
        _next = next;
    }

    public Task Invoke(HttpContext context)
    {
        var path = context.Request.Path.Value;
        if (_wwwrootDirs.Any(dir => path.IndexOf($"/{dir}", StringComparison.OrdinalIgnoreCase) == 0))
        {
            context.Request.Path = new PathString(context.Request.Path.Value.ToLower());
        }

        return _next(context);
    }
}

Startup.Configure 中添加中间件的代码如下：

var wwwrootDirs = Directory.GetDirectories(env.WebRootPath)
    .Select(dir => Path.GetFileName(dir));
app.UseMiddleware<LowercaseUrlMiddleware>(wwwrootDirs);
app.UseStaticFiles();