If you don't use prefix for client_service, it ignores ":only" argument

Let's assume we have an entity working with two APIs. For example:

```ruby
 class File < ApiStruct::Entity
 client_service CdnClient, only: :show
 client_service RestClient, only: :index

 attr_entity :size, :url, :uuid, :source
 end
```
If you call `File.show` it will try to call `RestClient.new.show` instead of `CdnClient.new.show`.