Copy HTTP_X_FORWARDED_PROTO to HTTPS so WordPress can detect is_ssl()

update-wp-config.php

//copy HTTP_X_FORWARDED_PROTO config to HTTPS var in $_SERVER
if((empty($_SERVER['HTTPS']) || $_SERVER['HTTPS'] == 'off') && $_SERVER['HTTP_X_FORWARDED_PROTO'] == 'https')
  $_SERVER['HTTPS'] = 'on';