Strange error using archive_open/4

Strange error when using `archive_open/4` with `Mode=write`:

```prolog
?- [library(archive)].
?- open('a.bz2', write, Out), archive_open(Out, write, Arch, [filter(bzip2),format(empty)]).
ERROR: option `format' does not exist in [filter(bzip2),format(empty)]
```

It's not clear to the programmer what is wrong here: option `format` does seem to be part of the options list.
