Select filter is not working

__Spec:__
```
activeadmin (1.4.3)
activeadmin-mongoid (0.8.0)
remote: https://github.com/activeadmin/activeadmin-mongoid.git
revision: ff45b6379cec574e687548b607cad225fd2cf403
branch: master
ransack-mongoid (0.1.0)
remote: https://github.com/activerecord-hackery/ransack-mongoid.git
revision: bd1935293d2f285fa7cf33d2c5c90d981bf67338
```

__Error__
`undefined method `+' for nil:NilClass`

__Reported line__
`activeadmin (1.4.3) lib/active_admin/inputs/filters/select_input.rb`

__Active admin setup:__

```
ActiveAdmin.register SellOrder do
actions :all, :except => [:new, :edit, :destroy]

index do
selectable_column
column :name
column :user
column "Domains" do |sell_order|
sell_order.domains.count
end
column :state
actions
end

filter :name
filter :state, as: :select, collection: SellOrder.state_machine.states.map(&:name)
filter :created_at

show do
...
end
end
```

__Investigation__
`activeadmin-mongoid` substitutes method [searchable_method_name](https://github.com/activeadmin/activeadmin/blob/master/lib/active_admin/inputs/filters/select_input.rb#L10) with broken equivalent, while original method itself works like a charm