Fix Date object unmarshalling

Notion API can return date from page fields in the form of "yyyy-mm-dd"
if the field configuration does not include time. The resulting string from the API
isn't accepted by the time.RFC3339 format and thus causes an unmarshalling
error.

The present change fixes it by attempting to parse it as such in case
the standard format fails. If it fails again, the error is returned.

object.go

@@ -1,6 +1,8 @@
 package notionapi
 
-import "time"
+import (
+	"time"
+)
 
 type ObjectType string
 
@@ -73,6 +75,27 @@ func (d Date) MarshalText() ([]byte, error) {
 	return []byte(d.String()), nil
 }
 
+func (d *Date) UnmarshalText(data []byte) error {
+	t, err := time.Parse(time.RFC3339, string(data))
+
+	// Because the API does not distinguish between datetime with a
+	// timezone and dates, we eventually have to try both.
+	if err != nil {
+		if _, ok := err.(*time.ParseError); !ok {
+			return err
+		} else {
+			t, err = time.Parse("2006-01-02", string(data)) // Date
+			if err != nil {
+				// Still cannot parse it, nothing else to try.
+				return err
+			}
+		}
+	}
+
+	*d = Date(t)
+	return nil
+}
+
 type File struct {
 	Name string `json:"name"`
 }

object_test.go

@@ -0,0 +1,35 @@
+package notionapi_test
+
+import (
+	"testing"
+
+	"github.com/jomei/notionapi"
+)
+
+func TestDate(t *testing.T) {
+	t.Run(".UnmarshalText", func(t *testing.T) {
+		var d notionapi.Date
+
+		t.Run("OK datetime with timezone", func(t *testing.T) {
+			data := []byte("1987-02-13T00:00:00.000+01:00")
+			err := d.UnmarshalText(data)
+			if err != nil {
+				t.Fatal(err)
+			}
+		})
+		t.Run("OK date", func(t *testing.T) {
+			data := []byte("1985-01-02")
+			err := d.UnmarshalText(data)
+			if err != nil {
+				t.Fatal(err)
+			}
+		})
+		t.Run("NOK", func(t *testing.T) {
+			data := []byte("1985")
+			err := d.UnmarshalText(data)
+			if err == nil {
+				t.Fatalf("expected an error, got none")
+			}
+		})
+	})
+}