Local DateTime support (#317)

tomltree_write.go

@@ -138,6 +138,8 @@ func tomlValueStringRepresentation(v interface{}, indent string, arraysOneElemen
 		return value.Format(time.RFC3339), nil
 	case LocalDate:
 		return value.String(), nil
+	case LocalDateTime:
+		return value.String(), nil
 	case nil:
 		return "", nil
 	}