fix: add json serialization support for option

Signed-off-by: Dr. Carsten Leue <carsten.leue@de.ibm.com>

either/either_test.go

@@ -11,6 +11,12 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
+func TestDefault(t *testing.T) {
+	var e Either[error, string]
+
+	assert.Equal(t, Of[error](""), e)
+}
+
 func TestIsLeft(t *testing.T) {
 	err := errors.New("Some error")
 	withError := Left[error, string](err)