我通过使用此问题答案中提供的技术解决了这个问题：

这是我上的课：

/// <summary>
/// Similiar to <see cref="JsonResult"/>, with
/// the exception that the <see cref="DataContract"/> attributes are
/// respected.
/// </summary>
/// <remarks>
/// Based on the excellent stackoverflow answer:
/// https://stackoverflow.com/a/263416/1039947
/// </remarks>
public class JsonDataContractActionResult : ActionResult
{
    /// <summary>
    /// Initializes a new instance of the class.
    /// </summary>
    /// <param name="data">Data to parse.</param>
    public JsonDataContractActionResult(Object data)
    {
        Data = data;
    }

    /// <summary>
    /// Gets or sets the data.
    /// </summary>
    public Object Data { get; private set; }

    /// <summary>
    /// Enables processing of the result of an action method by a 
    /// custom type that inherits from the ActionResult class. 
    /// </summary>
    /// <param name="context">The controller context.</param>
    public override void ExecuteResult(ControllerContext context)
    {
        if (context == null)
            throw new ArgumentNullException("context");

        var serializer = new DataContractJsonSerializer(Data.GetType());

        string output;
        using (var ms = new MemoryStream())
        {
            serializer.WriteObject(ms, Data);
            output = Encoding.UTF8.GetString(ms.ToArray());
        }

        context.HttpContext.Response.ContentType = "application/json";
        context.HttpContext.Response.Write(output);
    }
}

用法：

    public ActionResult TestFunction()
    {
        var testObject = new TestClass();
        return new JsonDataContractActionResult(testObject);
    }

我还必须修改初始类：

// -- The DataContract property was added --
[DataContract]
public class JsonTreeNode
{
    [DataMember(Name = "title")]
    public string Title { get; set; }

    [DataMember(Name = "isFolder")]
    public bool IsFolder { get; set; }

    [DataMember(Name = "key")]
    public string Key { get; set; }

    [DataMember(Name = "children")]
    public IEnumerable<JsonTreeNode> Children { get; set; }

    [DataMember(Name = "select")]
    public bool SelectedOnInit { get; set; }
}