正文中的HTTP发布中的Unicode字符不起作用

Question

我正在尝试使用WebClient调用发布包含Unicode nDash字符（–）的字符串：

{
    "data": "This is a test – string"
}

当我尝试发布时，WebClient收到空值。

这是我的WebClient呼叫：

using (IDisposable context = System.Web.Hosting.HostingEnvironment.Impersonate()) {
    var data = false;
    using (var client = new WebClientCall()) {
        client.BaseAddress = "https://BaseAddress"
        client.Headers.Clear();
        client.Headers.Add(HttpRequestHeader.ContentType, "application/json");
        client.Credentials = CredentialCache.DefaultCredentials;

        var dataString = JsonConvert.SerializeObject(messageContents);
        var json = client.UploadString($"api/data", "POST", dataString);

        data = JsonConvert.DeserializeObject<bool>(json, new JsonSerializerSettings() {
            ContractResolver = new CamelCasePropertyNamesContractResolver()
        });

        return data;
    }
}

private class WebClientCall : WebClient {
    protected override WebRequest GetWebRequest(Uri address) {
        HttpWebRequest webRequest = base.GetWebRequest(address) as HttpWebRequest;
        if (webRequest != null) {
            webRequest.Timeout = (int)TimeSpan.FromMinutes(10).TotalMilliseconds;
            webRequest.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
        }
        return (WebRequest)webRequest;
    }
}

有人可以帮我在WebClient呼叫中发送非ASCII字符吗？