[#111] Fix expiration epoch headers

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

downloader/download_test.go

@@ -0,0 +1,23 @@
+package downloader
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestSystemBackwardTranslator(t *testing.T) {
+	input := []string{
+		"__NEOFS__EXPIRATION_EPOCH",
+		"__NEOFS__RANDOM_ATTR",
+	}
+	expected := []string{
+		"Neofs-Expiration-Epoch",
+		"Neofs-Random-Attr",
+	}
+
+	for i, str := range input {
+		res := systemBackwardTranslator(str)
+		require.Equal(t, expected[i], res)
+	}
+}