HttpWebRequest 请求带OAuth2 授权的webapi

OAuth 2.0注意事项： 
1、 获取access_token时，请使用POST

1  private static string GetAuthorization(string username, string password)
2         {
3             string authorization = string.Format("{0}:{1}", username, password);
4 
5             return "Basic " + Convert.ToBase64String(new ASCIIEncoding().GetBytes(authorization));
6         }