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

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

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

pkg/object/attribute.go

@@ -17,6 +17,10 @@ func NewAttributeFromV2(aV2 *object.Attribute) *Attribute {
 // NewAttribute creates and initializes blank Attribute.
 //
 // Works similar as NewAttributeFromV2(new(Attribute)).
+//
+// Defaults:
+// 	- key: "";
+// 	- value: "".
 func NewAttribute() *Attribute {
 	return NewAttributeFromV2(new(object.Attribute))
 }

pkg/object/attribute_test.go

@@ -64,3 +64,19 @@ func TestAttribute_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNewAttribute(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		a := NewAttribute()
+
+		// check initial values
+		require.Empty(t, a.Key())
+		require.Empty(t, a.Value())
+
+		// convert to v2 message
+		aV2 := a.ToV2()
+
+		require.Empty(t, aV2.GetKey())
+		require.Empty(t, aV2.GetValue())
+	})
+}