Use general select type for associations

Now users shouldn't think about `collection` or `belongs_to` associations and filter type.
In case, when user manually sets (overwrites default `collection`) type `select`:
```erb
<%= filter_form_for @q do |f| %>
  <%= f.filter_input :parents, as: :select # collection %>
<% end %>
```
It automatically will find out is it collection or belongs_to association itself.