toJson () {
    const json = {}
    Object.keys(this).reduce((json, key) => {
      if (!key.startsWith('_')) {
        json[key] = this[key]
      }
      return json
    }, json)
    return json
  }
Convert page's metadata to JSON, note that all fields beginning
with an underscore will not be serialized.

@returns {object}
@api public