field_presave event always processes all fields

The field_presave events lets the user select which fields to act on. This functionality currently does not work, causing all fields to be processed.

The problem lies in the $preset->events_settings['field_presave_allowed_fields'] array, which has a key for all listed fields, whether they are enabled or not. The value for this key is (integer)0 or (string)[field_name], so a solution is to check if the value is indeed a string.

function _media_derivatives_field_presave_validation($file, $preset, $context) {
  return (in_array($context['field'], $preset->events_settings['field_presave_allowed_fields']) &&
   is_string($preset->events_settings['field_presave_allowed_fields'][$context['field']]) );
}