[#302] pkg/xheader: Document default values set in NewXHeader

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

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

pkg/xheader.go

@@ -15,6 +15,10 @@ func NewXHeaderFromV2(v *session.XHeader) *XHeader {
 }
 
 // NewXHeader creates, initializes and returns blank XHeader instance.
+//
+// Defaults:
+//  - key: "";
+//  - value: "".
 func NewXHeader() *XHeader {
 	return NewXHeaderFromV2(new(session.XHeader))
 }

pkg/xheader_test.go

@@ -40,3 +40,19 @@ func TestXHeader_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNewXHeader(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		xh := NewXHeader()
+
+		// check initial values
+		require.Empty(t, xh.Value())
+		require.Empty(t, xh.Key())
+
+		// convert to v2 message
+		xhV2 := xh.ToV2()
+
+		require.Empty(t, xhV2.GetValue())
+		require.Empty(t, xhV2.GetKey())
+	})
+}