.NET Core WebApi中返回 json 数据首字母大小写问题

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc().AddJsonOptions(opt =>
    {
       opt.SerializerSettings.ContractResolver = new Newtonsoft.Json.Serialization.DefaultContractResolver();//json字符串大小写原样输出
    });
}