Bug 17755: (QA followup) Return $self when appropriate

Bug 17755: (QA followup) Return $self when appropriate

As failure situations raise exceptions that should be handled outside
the object class, methods should return $self so successive calls can be
chained nicely.

This patch makes methods return $self and adjusts the tests to reflect
this change.

Make sure tests pass:
- Run:
  $ prove t/db_dependent/Koha/Patron/Attribute/Types.t
=> SUCCESS: Tests return green
- Sign off :-D

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>