Fix handling of HEAD requests in WP_REST_Server

[danielbachhuber]

HEAD request handling has been broken in all of the 2.x betas because, surprise, there was no test coverage for it.

The intended behavior for a HEAD request is to use the GET callback method. See how this is ​handled in 1.2.4

public function dispatch() { switch ( $this->method ) { case 'HEAD': case 'GET': $method = self::METHOD_GET; break; 

We should do equivalent in the REST server infrastructure.

[markjaquith]

Looks good. Asking for secondary review.

[helen]

Go forth.

[markjaquith]

In 35758:

Route HEAD API requests through the GET callback method

fixes #34837
props danielbachhuber