Drain response body properly for http connection pool (#6415)

Currently Go http connection pool was not being properly
utilized leading to degrading performance as the number
of concurrent requests increased.

As recommended by Go implementation, we have to drain the
response body and close it.

cmd/gateway/oss/gateway-oss.go

@@ -21,7 +21,6 @@ import (
 	"encoding/xml"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"net/http"
 	"strconv"
 	"strings"
@@ -816,11 +815,8 @@ func ossListObjectParts(client *oss.Client, bucket, object, uploadID string, par
 		return lupr, err
 	}
 
-	defer func() {
-		// always drain output (response body)
-		io.CopyN(ioutil.Discard, resp.Body, 512)
-		resp.Body.Close()
-	}()
+	// always drain output (response body)
+	defer minio.CloseResponse(resp.Body)
 
 	err = xml.NewDecoder(resp.Body).Decode(&lupr)
 	if err != nil {