Wrong sender address when run wp-cron from command-liine

[NoAnyLove]

For some reasons (php-fpm request timeout, more accurate cron, etc.), I disable wp-cron by adding define('DISABLE_WP_CRON', true); in wp-config.php, and running wp-cron by adding */10 * * * * /usr/local/bin/php /data/wwwroot/blog/wp-cron.php in crontab.

This causes varaible $_SERVER['SERVER_NAME'] unset when wp-cron triggered from crontab. So the Scheduled Backup will send email with sender emaill address as wordpress without domain name. As a result, the email system simply treat it as a spam.

I think it may be better to improve the way of getting sitename. If the $_SERVER['SERVER_NAME'] is not available, we can sue site_url() function instead.

[NoAnyLove]

I tried to fix this in pull#18, it works on my blog with version 4.4.1. This is my first time writing php code, not sure if there is something wrong with my code.

I also try to make it compatible with versions below 3.0.0, but I didn't test it on an old version WordPress.