[#132] v2/object: Implement stable unmarshaler on Object

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

v2/object/marshal.go

@@ -2,6 +2,7 @@ package object
 
 import (
 	"github.com/nspcc-dev/neofs-api-go/util/proto"
+	object "github.com/nspcc-dev/neofs-api-go/v2/object/grpc"
 )
 
 const (
@@ -440,6 +441,21 @@ func (o *Object) StableSize() (size int) {
 	return size
 }
 
+func (o *Object) StableUnmarshal(data []byte) error {
+	if o == nil {
+		return nil
+	}
+
+	objGRPC := new(object.Object)
+	if err := objGRPC.Unmarshal(data); err != nil {
+		return err
+	}
+
+	*o = *ObjectFromGRPCMessage(objGRPC)
+
+	return nil
+}
+
 func (r *GetRequestBody) StableMarshal(buf []byte) ([]byte, error) {
 	if r == nil {
 		return []byte{}, nil

v2/object/marshal_test.go

@@ -688,3 +688,15 @@ func generateRangeHashResponseBody(n int) *object.GetRangeHashResponseBody {
 
 	return resp
 }
+
+func TestObject_StableUnmarshal(t *testing.T) {
+	obj := generateObject("some data")
+
+	data, err := obj.StableMarshal(nil)
+	require.NoError(t, err)
+
+	obj2 := new(object.Object)
+	require.NoError(t, obj2.StableUnmarshal(data))
+
+	require.Equal(t, obj, obj2)
+}