Fix build failures

Author: cnblogs-dudu

src/OssClient.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using System.Net;
 using System.Net.Http;
 using System.Threading.Tasks;
 using Cuiliang.AliyunOssSdk.Api;

src/Request/ServiceCaller.cs

@@ -16,7 +16,6 @@ namespace Cuiliang.AliyunOssSdk.Request
     /// </summary>
     public class ServiceCaller
     {
-        private readonly static HttpClient _client = new HttpClient();
         private RequestContext _requestContext;
         private readonly HttpClient _client;