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)>'
```
The last page of the first collection is only loaded into memory when
it is being viewed. If it isn't loaded into memory, the `#size` call
triggers a count query.
This `#count` would generate an invalid query if our custom preloaded
counts are included by adding a separate `as count_column` alias on
top of the count aliases. Removing the selects in this case will make
sure a valid `COUNT(*)` is generated.
