Address ISBN issues in T-PAC

Address ISBN issues in T-PAC

A few problems fixed here:

* Refer to args.isbns (plural) consistently; at times we were referring
  to args.isbn which did not exist
* Grab the textContent of the SFX ISBN node rather than the raw
  XML::LibXML Element
* Use an assignment operation when regex'ing the SFX ISBN, as the
  in-place was interpreted as a "print me" stmt and displaying the raw
  SFX ISBN.

Signed-off-by: Dan Scott <dscott@laurentian.ca>