Bug 24606: Fix encoding issues on decoded content

Bug 24606: Fix encoding issues on decoded content

This patch makes the `decoded_contents` method UTF-8 encode the data
before calling the relevant decode_json, as expected by the JSON library
[1].

To test:
1. Apply the regression tests patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Koha/Item/Template.t
=> FAIL: Tests fail!
3. Apply this patch
4. Repeat 2
=> SUCCESS: Tests pass!

[1] `Any decoding or encoding (e.g. to UTF-8 or UTF-16) needs to be done
yourself, e.g. using the Encode module.` https://metacpan.org/pod/JSON

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>