code: @Override
    public Long decr(final byte[] key) {
      checkIsInMultiOrPipeline();
      client.decr(key);
      return client.getIntegerReply();
    }
docs: Decrement the number stored at key by one. If the key does not exist or contains a value of a
  wrong type, set the key to the value of "0" before to perform the decrement operation.
  <p>
  INCR commands are limited to 64 bit signed integers.
  <p>
  Note: this is actually a string operation, that is, in Redis there are not "integer" types.
  Simply the string stored at the key is parsed as a base 10 64 bit signed integer, incremented,
  and then converted back as a string.
  <p>
  Time complexity: O(1)
  @see #incr(byte[])
  @see #incrBy(byte[], long)
  @see #decrBy(byte[], long)
  @param key
  @return Integer reply, this commands will reply with the new value of key after the increment.