Fix wrong if-statement in get_items

Fixes a wrong if-statement in get_items that lead to an always empty list.
The statement checked for the dictionary key 'collections' which is never there and always returns false.
The correct way is to check for the 'items' key.

Author: ckubgi

dspace_rest_client/client.py

@@ -844,7 +844,7 @@ def get_items(self):
         items = list()
         if '_embedded' in r_json:
             # This is a list of items
-            if 'collections' in r_json['_embedded']:
+            if 'items' in r_json['_embedded']:
                 for item_resource in r_json['_embedded']['items']:
                     items.append(Item(item_resource))
         elif 'uuid' in r_json: