[#219] Return ETag in quotes

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/data/tree.go

@@ -93,9 +93,17 @@ type PartInfo struct {
 
 // ToHeaderString form short part representation to use in S3-Completed-Parts header.
 func (p *PartInfo) ToHeaderString() string {
+	// ETag value contains SHA256 checksum which is used while getting object parts attributes.
 	return strconv.Itoa(p.Number) + "-" + strconv.FormatUint(p.Size, 10) + "-" + p.ETag
 }
 
+func (p *PartInfo) GetETag(md5Enabled bool) string {
+	if md5Enabled && len(p.MD5) > 0 {
+		return p.MD5
+	}
+	return p.ETag
+}
+
 // LockInfo is lock information to create appropriate tree node.
 type LockInfo struct {
 	id uint64