[#288] pkg/eacl: Document default values set in NewFilter

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

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

pkg/acl/eacl/filter.go

@@ -108,6 +108,12 @@ func (k filterKey) String() string {
 }
 
 // NewFilter creates, initializes and returns blank Filter instance.
+//
+// Defaults:
+//  - header type: HeaderTypeUnknown;
+//  - matcher: MatchUnknown;
+//  - key: "";
+//  - value: "".
 func NewFilter() *Filter {
 	return NewFilterFromV2(new(v2acl.HeaderFilter))
 }

pkg/acl/eacl/filter_test.go

@@ -3,6 +3,7 @@ package eacl
 import (
 	"testing"
 
+	"github.com/nspcc-dev/neofs-api-go/v2/acl"
 	v2acl "github.com/nspcc-dev/neofs-api-go/v2/acl"
 	"github.com/stretchr/testify/require"
 )
@@ -66,4 +67,22 @@ func TestFilter_ToV2(t *testing.T) {
 
 		require.Nil(t, x.ToV2())
 	})
+
+	t.Run("default values", func(t *testing.T) {
+		filter := NewFilter()
+
+		// check initial values
+		require.Empty(t, filter.Key())
+		require.Empty(t, filter.Value())
+		require.Equal(t, HeaderTypeUnknown, filter.From())
+		require.Equal(t, MatchUnknown, filter.Matcher())
+
+		// convert to v2 message
+		filterV2 := filter.ToV2()
+
+		require.Empty(t, filterV2.GetKey())
+		require.Empty(t, filterV2.GetValue())
+		require.Equal(t, acl.HeaderTypeUnknown, filterV2.GetHeaderType())
+		require.Equal(t, acl.MatchTypeUnknown, filterV2.GetMatchType())
+	})
 }