Method constructor

test/driver.test.js:41–43

(key, options)

      SchemaTypes: {
        DateString: class DateString extends mongoose.SchemaType {
          constructor(key, options) {
            super(key, options, 'DateString');
          }

          cast(val) {
            if (typeof val !== 'string' || !/^\d{4}-\d{2}-\d{2}$/.test(val)) {