如何使用HttpClient.PostAsync发送XML内容？

我正在尝试实现此其余api：

public async Task<bool> AddTimetracking(Issue issue, int spentTime)
{
    // POST /rest/issue/{issue}/timetracking/workitem
    var workItem = new WorkItem(spentTime, DateTime.Now);
    var httpContent = new StringContent(workItem.XDocument.ToString());
    var requestUri = string.Format("{0}{1}issue/{2}/timetracking/workitem", url, YoutrackRestUrl, issue.Id);
    var respone = await httpClient.PostAsync(requestUri, httpContent);
    if (!respone.IsSuccessStatusCode)
    {
        throw new InvalidUriException(string.Format("Invalid uri: {0}", requestUri));
    }

    return respone.IsSuccessStatusCode;
}

workItem.XDocument 包含以下元素：

<workItem>
  <date>1408566000</date>
  <duration>40</duration>
  <desciption>test</desciption>
</workItem>

我从API中收到一个错误： Unsupported Media Type

我真的不知道该如何解决，非常感谢您的帮助。如何使用HTTP通过HTTP POST URI 封送 XML文件HttpClient？