gateway-s3: Avoid x2 double quotes in ListParts (#4295)

ListParts response returns doubled double quotes in ETag field.
This commit cleans ETag when receiving it from minio client to
fix the issue.

cmd/gateway-s3.go

@@ -449,7 +449,7 @@ func (l *s3Gateway) CopyObjectPart(srcBucket string, srcObject string, destBucke
 func fromMinioClientObjectPart(op minio.ObjectPart) PartInfo {
 	return PartInfo{
 		Size:         op.Size,
-		ETag:         op.ETag,
+		ETag:         canonicalizeETag(op.ETag),
 		LastModified: op.LastModified,
 		PartNumber:   op.PartNumber,
 	}