uses sets instead of arrays for associations

In JRuby on Rails with Puma in production mode with config.threadsafe!, a Her::Model was getting loaded twice, so the associations in that model were added twice and there is no check for duplicate associations in Her::Model::Associations. This resulted in unexpected behavior when parsing has_many associations. association_class.parse gets invoked twice for the same association and second time the data argument passed contains Her::Association object instead of Hash. Hence it sets the attributes of association class instance as nil.