mrp_microdata_aggregate_rating_attributes.php

<?php
/**
 * Adds itemprop=aggregateRating
 * 
 * @param unknown $microdata
 * @param unknown $post_id
 * @return string
 */
function mrp_microdata_aggregate_rating_attributes( $microdata, $post_id ) {
	return 'itemprop="aggregateRating" itemscope itemtype="http://schema.org/AggregateRating"';
}
add_filter( 'mrp_microdata_aggregate_rating_attributes', 'mrp_microdata_aggregate_rating_attributes', 10, 2 );