This fixes #80

It seems to be that JR always passes a hash to the relation_name accessors, with the context under key :context

https://github.com/cerebris/jsonapi-resources/blob/v0.9.0/lib/jsonapi/resource.rb#L250

Let's just use that everywhere in jsonapi-authorization, too.

It's a bit odd that JR passes way more information down the same relation_name when in the scope of processor, but that doesn't seem to be important to us. Documentation only refers to fetching the context from the options, and AFAICT, doesn't tell about what else is in the options hash:

http://jsonapi-resources.com/v0.9/guide/resources.html#Options

class BookResource < JSONAPI::Resource
  # Only book_admins may see unapproved comments for a book. Using
  # a lambda to select the correct relation on the model
  has_many :book_comments, relation_name: -> (options = {}) {
    context = options[:context]
    current_user = context ? context[:current_user] : nil
    unless current_user && current_user.book_admin
      :approved_book_comments
    else
      :book_comments
    end
  }
  ...
end