https-srcset-fix.php

<?php
/* The new responsive image feature in wordpress 4.4 causes image sourceset attribuet
  to insert images as HTTP rather than HTTPS.  This completely breaks images on sites where
  the site is loaded ( and in sometimes force-loaded) to be served via HTTPS
  The following plugin changes these images to be HTTPS if the site is served via HTTPS
  */
if ( is_ssl() ) {
  add_filter('wp_calculate_image_srcset', 'psu_https_srcset_fix' , 100 , 5);
}
function psu_https_srcset_fix( $sources, $size_array, $image_src, $image_meta, $attachment_id  ){
  $http_site_url = get_site_url( get_current_blog_id(), '/', 'http' );
  $https_site_url =  get_site_url( get_current_blog_id(), '/', 'https' );
  foreach( $sources as &$source ) {
    if (strpos($source['url'], $http_site_url) !== false ){
      $source['url'] = str_replace($http_site_url, $https_site_url, $source['url'] );
    }
  }
  return $sources;
}