Bug 25774: Handle utf8 chars in REST API queries

Bug 25774: Handle utf8 chars in REST API queries

We must utf8 encode the string before json decoding it.

Test plan:
Use "❤" and play with the library search (bug 25288) and Postman to
generate queries using it.

This patch prevents 500:
[2020/06/16 14:11:37] [ERROR] GET /api/v1/libraries: unhandled exception (Mojo::Exception)<<Wide character in subroutine entry at /kohadevbox/koha/Koha/REST/Plugin/Objects.pm line 107.>> Koha::REST::Plugin::Exceptions::__ANON__ /kohadevbox/koha/Koha/REST/Plugin/Exceptions.pm (73)

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>