Hide debug output for REST or Ajax requests

[jom]

Fixes Automattic/jetpack#25583

Prevents debug output on XHR or REST API requests.

[stodorovic]

I just looked this PR. Function wp_super_cache_query_vars is called before wp_cache_add_to_buffer. So it's safe to we use isset( $wp_super_cache_query[ 'is_rest' ] ).

I think that's better to avoid repeating same code snippet and alternative could be:

if ( strpos( $buffer, '<html' ) === false ||
     isset( $wp_super_cache_query[ 'is_rest' ] ) ||
     isset( $wp_super_cache_query[ 'is_feed' ] )
) {

Maybe we can keep comments for is_feed, I'm not sure. Also first condition is true in almost all cases (for XML files).

Related to AJAX request, we don't need anything special because output buffer isn't created. Also, I've created PR #384 and function wpsc_is_backend which covers more cases - cron requests, xlmrpc, wp-login, wp-cli and various PHP scripts.