Relationships on subclasses don't adhere to model name

A LoadError is thrown when trying to load a resource model with a namespaced custom model name that inherits from a abstract resource model that declares a relation.

Verified in versions 0.8.0.beta2 and 0.7.0

Modules:

module API
  module V2
    module Packages
      class PackageResource < BaseResource
        abstract
        has_many :venues
      end
    end
  end
end

module API
  module V2
    module Packages
      class MeetingPackageResource < PackageResource
        model_name 'Packages::MeetingPackage'
        has_many :services, class_name: 'MeetingPackageService'
      end
    end
  end
end

Path to meeting package module:

app/models/packages/meeting_package.rb

Error stack trace:

/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:495:in `load_missing_constant': Unable to autoload constant MeetingPackage, expected /app/models/packages/meeting_package.rb to define it (LoadError)
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/dependencies.rb:184:in `const_missing'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/inflector/methods.rb:261:in `const_get'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/inflector/methods.rb:261:in `block in constantize'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/inflector/methods.rb:259:in `each'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/inflector/methods.rb:259:in `inject'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/inflector/methods.rb:259:in `constantize'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/inflector/methods.rb:304:in `safe_constantize'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/activesupport-4.2.7.1/lib/active_support/core_ext/string/inflections.rb:77:in `safe_constantiz
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:934:in `_model_class'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:974:in `block in _add_relationship'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:970:in `each'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:970:in `_add_relationship'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:411:in `block in inherited'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:408:in `each_value'
/.rbenv/versions/2.2.4/lib/ruby/gems/2.2.0/gems/jsonapi-resources-0.8.0.beta2/lib/jsonapi/resource.rb:408:in `inherited'
/app/resources/api/v2/packages/meeting_package_resource.rb:4:in `<module:Packages>'
/app/resources/api/v2/packages/meeting_package_resource.rb:3:in `<module:V2>'
/app/resources/api/v2/packages/meeting_package_resource.rb:2:in `<module:API>'
/app/resources/api/v2/packages/meeting_package_resource.rb:1:in `<top (required)>'
...

Reproducir el caso de herencia con namespaces del issue, empezando en lib/jsonapi/resource.rb alrededor de _model_class, _add_relationship e inherited; compararlo con app/resources/api/v2/packages/meeting_package_resource.rb y app/models/packages/meeting_package.rb. Se considera completado cuando el recurso se carga sin el LoadError indicado y la relación venues heredada sigue siendo válida, con una prueba de regresión que cubra el caso.