Thing: Normalize int fields to be integers

[ocean90]

There's currently a unit test failure:

1) GP_Test_Thing_Translation_set::test_copy_translations_from_should_copy_into_empty_set
/tmp/wordpress/tests/phpunit/includes/abstract-testcase.php:644
/tmp/wordpress/src/wp-content/plugins/GlotPress-WP/tests/phpunit/testcases/tests_things/test_thing_translation_set.php:15

This is because $destination_set_translations[0]->translation_set_id has the value "2" (string) while $destination_set->id is 2 (integer).