def required(key)
      raise_error(type: "required", key: key) unless validator.datas.has_key?(key)
    end
def required(key)
      raise_error(type: "required", key: key) unless validator.datas.has_key?(key)
    end
Checks if a required key is present in provided datas.
 @param [Object] key the key of which check the presence.
 @raise [ArgumentError] if the key is not present.