Use default identifier in queued_search as can't handle others anyway

queued_search needs ids to be in form `site.model.pk`, which is the default haystack ID. However, the code is making use of `get_identifier` method which actually checks if a custom identifier method has been specified. In my case, I had one, and it returned ID's in the form of `model:pk` which is fine with Haystack but breaks everything in queued_search. This fix makes sure queued_search works while still using the id's that user wants in the index itself (as that is picked by Haystack in the end anyway, as long as you pass an instance of an object and not a string).