/// <summary>
 /// 确保httpContent的内容未被缓存
 /// 已被缓存则抛出HttpContentBufferedException
 /// </summary>
 /// <param name="httpContent"></param>
 /// <exception cref="HttpContentBufferedException"></exception>
 public static void EnsureNotBuffered(this HttpContent httpContent)
 {
     if (httpContent.IsBuffered() == true)
     {
         throw new HttpContentBufferedException();
     }
 }