[#302] pkg/checksum: Document default values set in NewChecksum

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

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

pkg/checksum.go

@@ -34,9 +34,13 @@ func NewChecksumFromV2(cV2 *refs.Checksum) *Checksum {
 	return (*Checksum)(cV2)
 }
 
-// NewVersion creates and initializes blank Version.
+// NewChecksum creates and initializes blank Checksum.
 //
-// Works similar as NewVersionFromV2(new(Version)).
+// Works similar as NewChecksumFromV2(new(Checksum)).
+//
+// Defaults:
+//  - sum: nil;
+//  - type: ChecksumUnknown.
 func NewChecksum() *Checksum {
 	return NewChecksumFromV2(new(refs.Checksum))
 }

pkg/checksum_test.go

@@ -115,3 +115,19 @@ func TestChecksum_ToV2(t *testing.T) {
 		require.Nil(t, x.ToV2())
 	})
 }
+
+func TestNewChecksum(t *testing.T) {
+	t.Run("default values", func(t *testing.T) {
+		chs := NewChecksum()
+
+		// check initial values
+		require.Equal(t, ChecksumUnknown, chs.Type())
+		require.Nil(t, chs.Sum())
+
+		// convert to v2 message
+		chsV2 := chs.ToV2()
+
+		require.Equal(t, refs.UnknownChecksum, chsV2.GetType())
+		require.Nil(t, chsV2.GetSum())
+	})
+}