[SalesForce] Update Opportunity Using rest API; Error : 400 bad request in response

Example for creating Opportunity :

curl https://na1.salesforce.com/services/data/v20.0/sobjects/Opportunity/0AASSFGSEID -H "Authorization: Bearer token" -H "Content-Type: application/json" -d @newrecord.json -X PATCH

Example request body newrecord.json file :

{ "Name":"FFNEw","CloseDate":"2015/02/04","StageName":"Prospecting","Probability":10 }

My ASP.net code :

using (WebClient client = new WebClient())
{
client.Headers.Add("Authorization", "Bearer " + token.access_token); client.Headers.Add("Content-Type", "application/json");
var request = (HttpWebRequest)(HttpWebRequest.Create(token.instance_url + "/services/data/v20.0/sobjects/Opportunity/0AASSFGSEID"));
request.Method = "PATCH";
using (var requestWriter = new StreamWriter(request.GetRequestStream()))
{
  requestWriter.Write(json);
  requestWriter.Flush();
  requestWriter.Close();
}
var response = request.GetResponse();
}

Getting Error 400 Bad Request in "request.GetResponse()".