Makes it possible to use current_user within serializers

Setting the scope to the `endpoint` makes it possible to use `current_user` within serializers.

Serializers must delegate `:current_user` to the scope, like this:

    class BaseSerializer < ActiveModel::Serializer
      delegate :current_user, to: :scope
    end

I personally use this `BaseSerializer` and then extend all my serializers from this one.