[#302] pkg/object: Document default values set in NewRange

Document field values of instance constructed via `NewRange`.
Assert the values in corresponding unit test.

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

pkg/object/range_test.go

@@ -1,9 +1,9 @@
 package object
 
 import (
-	"github.com/nspcc-dev/neofs-api-go/v2/object"
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/object"
 	"github.com/stretchr/testify/require"
 )
 
@@ -40,3 +40,19 @@ func TestRange_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNewRange(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		r := NewRange()
+
+		// check initial values
+		require.Zero(t, r.GetLength())
+		require.Zero(t, r.GetOffset())
+
+		// convert to v2 message
+		rV2 := r.ToV2()
+
+		require.Zero(t, rV2.GetLength())
+		require.Zero(t, rV2.GetOffset())
+	})
+}