[#302] pkg/signature: Document default values set in NewSignature

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

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

pkg/signature.go

@@ -17,6 +17,10 @@ func NewSignatureFromV2(sV2 *refs.Signature) *Signature {
 // NewSignature creates and initializes blank Signature.
 //
 // Works similar as NewSignatureFromV2(new(Signature)).
+//
+// Defaults:
+//  - key: nil;
+//  - signature: nil.
 func NewSignature() *Signature {
 	return NewSignatureFromV2(new(refs.Signature))
 }

pkg/signature_test.go

@@ -1,9 +1,9 @@
 package pkg
 
 import (
-	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
 	"github.com/stretchr/testify/require"
 )
 
@@ -48,3 +48,19 @@ func TestSignature_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNewSignature(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		sg := NewSignature()
+
+		// check initial values
+		require.Nil(t, sg.Key())
+		require.Nil(t, sg.Sign())
+
+		// convert to v2 message
+		sgV2 := sg.ToV2()
+
+		require.Nil(t, sgV2.GetKey())
+		require.Nil(t, sgV2.GetSign())
+	})
+}