Bug 27420: Use object accessor for $attr_type->class()

Bug 27420: Use object accessor for $attr_type->class()

We introduced a bug in the patron attribute forms with bug 5161.

Test plan
1/ Create two PA_CLASS authorized values
2/ Create two corresponding patron attribute types referencing the above
classes.
3/ Edit a patron, both attributes should appear within their own
fieldsets at the bottom of the member entry form.
4/ Set a value for the first of the two patron attributes and save
5/ Edit the patron again, note that the first attribute no longer
resides within it's own fieldset
6/ Apply the patch
7/ Edit the patron again, note that the first attribute now resides
inside it's own fieldset again
8/ Signoff

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>