Fix specs, as we can have array in :identifier

to put each identifier from the array to
the result(returned in api_feature_identifiers)

spec/lib/api/api_config_spec.rb

@@ -19,7 +19,7 @@
 
       let(:api_feature_identifiers) do
         collection_settings.each_with_object(Set.new) do |(_, cfg), set|
-          set.add(cfg[:identifier]) if cfg[:identifier]
+          Array(cfg[:identifier]).each { |id| set.add(id) }
           keys = [:collection_actions, :resource_actions, :subcollection_actions, :subresource_actions]
           Array(cfg[:subcollections]).each do |s|
             keys << "#{s}_subcollection_actions" << "#{s}_subresource_actions"