Bug 17767: Make Koha::Patron::Modification handle extended attributes

Bug 17767: Make Koha::Patron::Modification handle extended attributes

This patch makes Koha::Patron::Modification aware of the new extended_attributes
column, which is expected to contain valid JSON data.

The ->store method is modified so it validates the field value (i.e. the content
is decoded using the JSON library) and raises a convenient exception in case of
failure.

This behaviour change is covered by the provided unit tests.

To test:
- Apply the patchset
- Run:
  $ prove t/db_dependent/Koha/Patron/Modifications.t
=> SUCCESS: Tests make sense, and they pass
- Sign off :-D

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>