AJAX hook URL returns 200 when bad/missing action

[ryanrolds]

When requesting /wp-admin/admin-ajax.php with an invalid/missing action the response has a status code of 200, which indicates a successful request. A 400 response with a reason would be more fitting.

[ayeshrajans]

Hi @ryanrolds. I'm not a core maintainer and I hope one of them will reply to this ticket soon. I also think an appropriate error response code makes sense.

I went ahead and made some changes so that invalid/forged admin-ajax.php requests die with a 400 error code. It will still print 0 to the screen (for compatibility).

Attaching patch. Link to tests: ​https://travis-ci.org/Ayesh/wordpress-develop/builds/251543055

[johnbillion]

In 41120:

Administration: Send an appropriate HTTP response status code when an invalid action is passed to admin-ajax.php.

Props ryanrolds, ayeshrajans

Fixes #41269