def set?(key)
      check_key!(key)
      expires!(key)
      @data.keys.include?(key.to_s)
    end
def set?(key)
      check_key!(key)
      expires!(key)
      @data.keys.include?(key.to_s)
    end
Public: Determines whether a value has been set for
 a given key.

 key - A String or Symbol representing the key.

 Returns a Boolean.