Support exportable entity types

For now, Search Api isn't handling indexing of exportables entity types.
I had an issue with the Countries module because of this.
Search API is always assuming that the entity identifier is an integer e.g (index_entity.inc) :

if (isset($entity_types[$type])) {
  $flat[$key]['type'] = search_api_nest_type('integer', $info['type']);
  $flat[$key]['entity_type'] = $type;
}

And _search_api_extract_entity_value called in search_api_extract_fields is using the getIdentifier method of the wrapper to return the value.
Finally the getIdentier method of the wrapper is returning the value of the id through the entity method :

  $key = isset($info['entity keys']['name']) ? $info['entity keys']['name'] : $info['entity keys']['id'];

The solution to solve this issue is to do the same test inside getFields() of the index_entity module.