Add test case from make_it_modular to filtered_ls

Here is a (really) small PR.

I was struggling to pass the `make_it_modular` challenge because it states that _"This problem is the same as the previous but introduces the concept of modules."_ and my program had a bug which was not reported in `filtered_ls`. I just added the right test case (one more file named like the tested file extension, `dat`) to make it fail in `filtered_ls` too.

Here is an extract of the solution that was passing in `filtered_ls` but not in `make_it_modular`: 

    files.forEach(function (file) {
      if (file.split('.').pop() === extension) {
        console.log(file);
      }
    });

If `file === extension`, then `file.split('.').pop() === extension` and this solution fails.

It failed only in `make_it_modular` because the test cases where:

    files = [
          'learnyounode.dat'
        , 'learnyounode.txt'
        , 'learnyounode.sql'
        , 'api.html'
        , 'README.md'
        , 'CHANGELOG.md'
        , 'LICENCE.md'
        , 'md'
        , 'data.json'
        , 'data.dat'
        , 'words.dat'
        , 'w00t.dat'
        , 'w00t.txt'
        , 'wrrrrongdat'
      ]