cannot save category b/c of 'site_id'

[Sixtease]

When trying to save a category, the requests fails with: core_category.site_id may not be NULL

reproduction (in javascript console):

$.ajax({
    url: "/api/r1/category/",
    type: 'post',
    data: '{"app_data":"{}","content":"","description":"","id":"1","slug":"vesele-zpravy","template":"category.html","title":"veselé zprávy"}',
    headers: {"Content-Type":"application/json"}
});

Please note that the category object doesn't have anything like ``core_category.site_id` when it is fetched from the backend.