HttpWebRequest POST 数据时请求头多了一行Expect: 100-continue，少了数据行

用HttpWebRequest POST数据时POST数据被截掉了。使用HTTPLook跟踪出来的HTTP头信息
POST /sp/SPLogin HTTP/1.1
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/x-shockwave-flash, */*
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322
Accept-Language: zh-cn
UA-CPU: x86
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 85
Expect: 100-continue
Connection: Keep-Alive
Host: test.com
Cookie: JSESSIONID=Ghdht2vhW2xTPcFRYG1ch1Xt1Kp81rRGvR11nlQNKW6l4MTZ1MHk!-1870340995正常情况应为:
POST /sp/SPLogin HTTP/1.1
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/x-shockwave-flash, */*
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322
Accept-Language: zh-cn
UA-CPU: x86
Accept-Encoding: gzip, deflate
Content-Type: application/x-www-form-urlencoded
Content-Length: 85
Connection: Keep-Alive
Host: admin.sh.monternet.com
Cookie: JSESSIONID=Ghdht2vhW2xTPcFRYG1ch1Xt1Kp81rRGvR11nlQNKW6l4MTZ1MHk!-1870340995selectAccount=SPPREREG&USER=907000&PASSWORD=111111&addCode=9146&Submit22=%B5%C7%C2%BC
以下是程序代码:
public static void GetWebContent(string url,CookieContainer cookies,string filename)
{
   HttpWebRequest request = null;
   HttpWebResponse response = null;
   CookieCollection cookieResponse = new CookieCollection();
   CookieContainer cookieRequest = new CookieContainer();
   Uri requestUri = new Uri(url);
   try
   {
      Cookie cookie = new Cookie();
      cookie.Name = "JSESSIONID";
      cookie.Value = "Ghdht2vhW2xTPcFRYG1ch1Xt1Kp81rRGvR11nlQNKW6l4MTZ1MHk!-1870340995";
      cookie.Path = "/";
      cookie.Domain =requestUri.Host;
      cookieRequest.Add(cookie);

      // 处理请求
      request = (HttpWebRequest) WebRequest.Create(new Uri(url));
      request.Accept ="image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/x-shockwave-flash, */*";
      request.UserAgent = "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.2; SV1; .NET CLR 1.1.4322";
      request.Headers.Add("Accept-Language: zh-cn");
      request.Headers.Add("UA-CPU: x86");
      request.Headers.Add("Accept-Encoding: gzip, deflate");
      request.Timeout = 20000;
      request.CookieContainer = cookieRequest;

      string data = "selectAccount=SPPREREG&USER=907000&PASSWORD=111111&addCode=9146&Submit22=%B5%C7%C2%BC";
      byte[] byteData = Encoding.Default.GetBytes(data);
      request.Method ="POST";
      request.ContentType = "application/x-www-form-urlencoded";
      request.ContentLength = byteData.Length;
      using (Stream requestStream = request.GetRequestStream())
      {
requestStream.Write(byteData,0,byteData.Length);
      }

      // 处理响应
      response = (HttpWebResponse)request.GetResponse();
      cookieResponse = response.Cookies ;
      byte[] buffer =  ResponseAsBytes(response);
      response.Close();      FileStream file = new FileStream(filename, FileMode.Create);
      file.Write(buffer,0,buffer.Length);

   
   }catch(Exception ex)
   {
      Log.Error("GetWeb:Url="+url + ex.Message,ex);
   }
}
那位知道原因吗？？