flurl升级之后没有FlurlNewtonsoftJsonSerializer

新建NewtonsoftJsonSerializer.cs

/// <summary>
/// ISerializer implementation based on Newtonsoft.Json.
/// Default serializer used in calls to GetJsonAsync, PostJsonAsync, etc.
/// </summary>
public class NewtonsoftJsonSerializer : IJsonSerializer
{
    private readonly JsonSerializerSettings _settings;

    /// <summary>
    /// Initializes a new instance of the <see cref="NewtonsoftJsonSerializer"/> class.
    /// </summary>
    /// <param name="settings">Settings to control (de)serialization behavior.</param>
    public NewtonsoftJsonSerializer(JsonSerializerSettings settings = null) {
        _settings = settings;
    }
    
    public string Serialize(object obj, Type type)
    {
        return JsonConvert.SerializeObject(obj, _settings);
    }

    public object Deserialize(string json, Type type)
    {
        return JsonConvert.DeserializeObject(json,type, _settings);
    }
}

代码修改为下面的即可