fix(sts): Fix the problem that the HttpResponse returned by getting STS is null

Author: zhenlei520

aliyun-net-sdk-core/Auth/Sts/AssumeRoleResponseUnmarshaller.cs

@@ -27,6 +27,7 @@ public static AssumeRoleResponse Unmarshall(UnmarshallerContext context)
         {
             var assumeRoleResponse = new AssumeRoleResponse();
 
+            assumeRoleResponse.HttpResponse = context.HttpResponse;
             assumeRoleResponse.RequestId = context.StringValue("AssumeRole.RequestId");
 
             var credentials = new AssumeRoleResponse.AssumeRole_Credentials();