Bug with numbered sheet filenames

Encountered an .xlsx that had the following files:

```
worksheets/sheet1.xml
worksheets/sheet3.xml
worksheets/sheet4.xml
```

Because the code uses the number in the filename minus one when assigning to an array, there's a blank spot at `[1]` (where sheet2.xml would belong).

Workaround (not sure it's a proper fix) I used was to exclude `nil` values from the array when iterating over it, using `:compact`.