use JSON rather than Storable for the OPAC search history cookie

use JSON rather than Storable for the OPAC search history cookie

To test:

Exercise the OPAC search history functionality, after
turning on the EnableOpacSearchHistory syspref:

- Clear the KohaOpacRecentSearches cookie
- As an anonymous user, conduct a variety of searches,
  including ones that include non-ASCII characters
- Check the search history and verified that all searches
  are listed
- Log into the OPAC
- Verify that current and past searches are listed in
  search history.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>
Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>