Wikibase API now reports the Qid for missing items

Whereas wikibase has reported -1 for deleted items, it is now reporting
the id of the item. e.g. Q404 is a deleted item, which wbgetentities now
returns a successful result of
	{"id": "Q404", "missing": ""}
A test was expecting -1, and now needs to expect Q404.

Change-Id: I713476bfbac61e1e7561cac94e1fec21a44d4db1