The manage subscription page does not preserve category

none of the links/action buttons on the manage subscription page preserve category.

To reproduce:

1. go to moodle_url/calendar/view.php?view=month&category=2 (where category is equal to a valid course category id)
2. Click on the 'Manage subscription' link
3. Click on the 'Export calendar' button
4. You'll see that the URL has lost the category parameter