Cron: 'Headers already sent' warning with `ALTERNATE_WP_CRON`

[dlh]

After [45560], a "Headers already sent" warning is generated when ALTERNATE_WP_CRON is enabled. The cause seems to be that spawn_cron() adds headers via wp_redirect() and then sends headers before loading wp-cron.php.

The attached patch would add a headers_sent() check before the two caching headers.

[jorbin]

In 46458:

Cron: Prevent sending headers when using ALTERNATE_WP_CRON

ALTERNATE_WP_CRON echos things which causes a "Headers already sent" warning. This adds a check to ensure headers haven't already been sent.

Previously: [45560]

See: #45560. Fixes #48283.
Props dlh.