Bug 18461: (bug 13757 follow-up) Do not mix decode_json and to_json

Bug 18461: (bug 13757 follow-up) Do not mix decode_json and to_json

Internal server error while approving OPAC-edited patron attributes
containing umlauts (äöü)

If a OPAC user enters text containing umlauts (äöü) in a patron atribute
and saves it, the Staff client will get an inernal server error in
members-update.pl (approving).

E.g. a text like 111ä222 leads to the following in plack error log:

malformed UTF-8 character in JSON string, at character offset 14

To test:
- Reproduce problem above
- Apply patch
- Verify that Home > Patron > Update records displays without error
and 'New value' displays properly
- Approve!

Also, run the tests:
- Run:
  $ sudo koha-shell kohadev
 k$ cd kohaclone
 k$ prove t/db_dependent/Koha/Patron/Modifications.t
=> FAIL: Tests explode due to encoding problems in JSON handling
- Apply the patch
- Run the tests again:
 k$ prove t/db_dependent/Koha/Patron/Modifications.t
=> SUCCESS: Tests pass!

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>