🎁 Configure OAI-PMH sets around collections

This commit will add collections as sets to the OAI-PMH feed.

Ref:
- #666

app/controllers/catalog_controller.rb

@@ -421,7 +421,8 @@ def self.created_field
       document: {
         limit: 100, # number of records returned with each request, default: 15
         set_fields: [ # ability to define ListSets, optional, default: nil
-          { label: 'collection', solr_field: 'isPartOf_ssim' }
+          { label: 'admin_set', solr_field: 'isPartOf_ssim' },
+          { label: 'collection', solr_field: 'member_of_collections_ssim' }
         ]
       }
     }