Grape::ActiveModelSerializers for models with compound names

    Grape::ActiveModelSerializers for models with compound names
      should generate the proper 'root' node for individual objects
      should generate the proper 'root' node for serialized arrays of objects

Just changed `#downcase` to `#underscore` in `lib/grape-active_model_serializers/formatter.rb`

And thanks for the library.  Much appreciated!