fix passing ID instead of id

[kevinpapst]

Depending on the PHP version the old code has different outcomes.
In my situation $post->id is always an empty string.

The correct field name is ID, see https://developer.wordpress.org/reference/classes/wp_post/

As you already use $post->ID in other places of the class, this fact is likely not new 😁

[benbalter]

Thanks for these fixes. While maintained, some of the code is 12+ years old at this point, and PHP and WordPress has changed a lot over that time.

[kevinpapst]

Yeah, I thought we fix it for the next person.
In the next few weeks more people will think about migrating their WP sites 😁