Use syntax from perlguts for testing objects

Use syntax from perlguts for testing objects

The following paragraph is in perlguts.pod:

   To check if you've got an object derived from a specific class you have
   to write:

       if (sv_isobject(sv) && sv_derived_from(sv, class)) { ... }

which does the right thing with magical things like tied scalars.

Signed-off-by: David Golden <dagolden@cpan.org>