Allow external ids with dashes (-)

Hi there, and again thank you for the amazing module :)

I was wondering if we could adjust the following:

  protected function doLoadMultiple(array $ids = NULL) {
    $entities = array();
    foreach ($ids as $id) {
      if (strpos($id, '-')) {
        list($bundle, $external_id) = explode('-', $id, 2);

And change the explode to only split on the first '-' and keep the remaining as the external_id. I needed this when working with CKAN which stores an unique id like so:

"id": "1aa53c2b-62da-4982-9fa7-c198d27bb908",

So my full string would be "ckan-1aa53c2b-62da-4982-9fa7-c198d27bb908"