szbl-fix-srcset-ssl.php

<?php
function szbl_ssl_srcset_fix( $src_list )
{
  foreach ( $src_list as $k => $src ) {
    $src_list[ $k ]['url'] = set_url_scheme( $src['url'], 'https' );
  }

  return $src_list;
}
add_filter( 'wp_calculate_image_srcset', 'szbl_ssl_srcset_fix' );