Allow for non id primary keys

[ychaker]

Hello,
Firstly, thank you for all of your hard work with this gem, it's been great using it.

I ran into an issue when using something other than the id as a primary key. Here's my resource definition:

module Api
  class Metro < ::Api::Base
    self.primary_key = :short_id

    # Overriding this to use the primary key instead of the id field
    #
    # When we represent this resource for serialization (create/update), we do so
    # with this implementation
    #
    # @return [Hash] Representation of this object as JSONAPI object
    def as_json_api(*)
      attributes.slice(:id, :type).tap do |h|
        if :id != self.class.primary_key
          h[:id] = attributes[self.class.primary_key]
        end

        relationships_for_serialization.tap do |r|
          h[:relationships] = self.class.key_formatter.format_keys(r) unless r.empty?
        end
        h[:attributes] = self.class.key_formatter.format_keys(attributes_for_serialization)
      end
    end
  end
end

without overriding as_json_api, the gem was sending the request to the proper url: /metros/:short_id but the payload looked like this: { data: { id: 1, type: 'metros' } } where the id value was the id field instead of the short_id so the id in the data payload did not match the url and the API was returning a 400 response because they identifiers weren't matching.

Overriding the as_json_api method works, but of course it would be better if the support for custom primary keys was baked into the gem.

Thanks.