Ensure that |get fingerprint| doesn't fail if the ID is not an array (issue 5599)

According to the [PDF specification](http://www.adobe.com/content/dam/Adobe/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#page=51) the `ID` should be an array. To prevent errors if the PDF file violates that part of the spec, check that the `ID` is actually an array.

Fixes #5599.