In Rails 5 the `per_page` argument came from `params` is String but an
Integer is expected.

This commit explicitly converts `per_page` to an Integer.
It fixes the errors like:

```
  1) Groups::ChildrenController GET #index for subgroups filtering children succeeds if multiple pages contain matching subgroups
     Failure/Error:
       hash[page] = second_collection.page(second_collection_page)
                      .per(per_page - paginated_first_collection(page).size)
                      .padding(offset)

     NoMethodError:
       undefined method `-' for "3":String
       Did you mean?  -@
     # ./lib/gitlab/multi_collection_paginator.rb:42:in `block in paginated_second_collection'
     # ./lib/gitlab/multi_collection_paginator.rb:46:in `paginated_second_collection'
     # ./lib/gitlab/multi_collection_paginator.rb:14:in `paginate'
     # ./app/finders/group_descendants_finder.rb:52:in `children'
     # ./app/finders/group_descendants_finder.rb:34:in `execute'
     # ./app/controllers/groups/children_controller.rb:36:in `setup_children'
     # ./app/controllers/groups/children_controller.rb:18:in `index'
     # ./lib/gitlab/i18n.rb:50:in `with_locale'
     # ./lib/gitlab/i18n.rb:56:in `with_user_locale'
     # ./app/controllers/application_controller.rb:351:in `set_locale'
     # ./spec/controllers/groups/children_controller_spec.rb:182:in `block (5 levels) in <top (required)>'
     # ./spec/spec_helper.rb:198:in `block (2 levels) in <top (required)>'
```