Fixes srcset URLs on WordPress Multisite installations when using Domain Mapping

fix_multisite_srcset.php

// Fix the URLS generated by media srcset from WP 4.4+
//
// When viewing the srcset URLs for images on a 4.4+ WordPress installation,
// it was showing my multisite subdomain URL rather than the domain chosen during Domain Mapping
//
// Add to your functions.php and modify the URL fields
//
// See: https://make.wordpress.org/core/2015/11/10/responsive-images-in-wordpress-4-4/ 
function fix_multisite_srcset( $sources ){
  foreach ( $sources as &$source ) {
    $sources[ $source['value'] ][ 'url' ] = str_replace('http://OLD-URL', 'http://ACTUAL-URL', $sources[ $source['value'] ][ 'url' ]);
	}
	return $sources;
}
add_filter( 'wp_calculate_image_srcset', 'fix_multisite_srcset' );