Fix output of ISBN-13 numbers beginning with 979.

 Fix output of ISBN-13 numbers beginning with 979.
 
 An EAN beginning with 979 (but not 9790 - those are ISMN's) are accepted
 as ISBN numbers, but they cannot be represented in the old, 10-digit ISBN
 format. They must be output in the new 13-digit ISBN-13 format. We printed
 out an incorrect value for those.
 
 Also add a regression test, to test this and some other basic functionality
 of the module.
 
 Patch by Fabien Coelho. This fixes bug #13442, reported by B.Z. Backpatch
 to 9.1, where we started to recognize ISBN-13 numbers.