Fix Rbac#apply_select extra_cols logic

Fixes and issue with Rbac#apply_select when `:extra_cols` exists and an
already existing `.select` is in the scope.  Currently, this will throw
an error (I guess).

lib/rbac/filterer.rb

@@ -656,7 +656,9 @@ def apply_scope(klass, scope)
     end
 
     def apply_select(klass, scope, extra_cols)
-      scope.select(scope.select_values.blank? ? klass.arel_table[Arel.star] : nil).select(extra_cols)
+      scope = scope.select(klass.arel_table[Arel.star]) if scope.select_values.blank?
+      scope = scope.select(extra_cols)                  if extra_cols.present?
+      scope
     end
 
     def get_belongsto_matches(blist, klass)