Bug 26832: Make Elasticsearch mappings export use UTF-8

Bug 26832: Make Elasticsearch mappings export use UTF-8

Script misc/search_tools/export_elasticsearch_mappings.pl allows to export current search engine configuration into a YAML file.
This export should use UTF-8 encoding, like other exports.

Test plan :
1) Go to Administration > Search engine configuration (Elasticsearch)
2) Edit a field label to use a diacrtic, for example local-number => Numéro
3) Save
4) Edit file etc/koha-conf.xml to enable 'elasticsearch_index_mappings'
5) Export mappings to file via misc/search_tools/export_elasticsearch_mappings.pl -t $MARCFLAVOUR
6) Reset memcached and plack
7) Back to Administration > Search engine configuration (Elasticsearch)
8) Click on 'Reset Mappings' and accept
9) Look at field 'local-number'
=> Without patch diacritic 'é' is broken
10) You may try with an emoji B-)

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>