code: @Override
    public Boolean exists(final byte[] key) {
      checkIsInMultiOrPipeline();
      client.exists(key);
      return client.getIntegerReply() == 1;
    }
docs: Test if the specified key exists. The command returns true if the key exists, otherwise false is
  returned. Note that even keys set with an empty string as value will return true. Time
  complexity: O(1)
  @param key
  @return Boolean reply, true if the key exists, otherwise false