Filtering SailThru Horizon meta tags

sailthru_horizon_meta_tags_filter.php

<?php
/**
 * Add support for CoAuthors Plus in SailThru plugin
 * @see http://getstarted.sailthru.com/wordpress-vip-plugin
 */
add_filter( 'sailthru_horizon_meta_tags', function( $horizon_tags, $post_object ){
	// If we are using the CoAuthors Plus plugin then we need to replace the value of sailthru.author
	// If there are multiple authors we use a comma seperated list
	if( is_single() && class_exists( 'coauthors_plus' ) && class_exists( 'Sailthru_Horizon' )   ){
		$coauthors = (array) wp_list_pluck ( (array) get_coauthors(), 'display_name' ); // array of authors
		$author_string = implode( ', ', array_unique( $coauthors ) ); // comma-seperated string list of authors
		if( $author_string )
			$horizon_tags['sailthru.author'] = esc_attr( $author_string );
	}
}, 10, 2 );

/**
 * Customize the  Sailthru.tags meta data to inlude the post's category and tag slugs
 * @see http://getstarted.sailthru.com/wordpress-vip-plugin
 */
add_filter( 'sailthru_horizon_meta_tags', function( $horizon_tags, $post_object ){
	// Replace Sailthru.tags with the posts of tag and category slugs
	$tags = (array) wp_list_pluck( (array) get_the_tags(), 'slug' );
	$cats = (array) wp_list_pluck( (array) get_the_category(), 'slug' );
	// cats and tags can have matching slugs so we merge and remove redundent slugs
	$tags_array = array_filter( array_unique( array_merge( $cats, $tags ) ) );
	$tag_string = implode( ', ', $tags_array );
	if( $tag_string ) {
		$horizon_tags['sailthru.tags'] =  esc_attr( $tag_string );
	}
	return $horizon_tags;
}, 10, 2 );