use proper checks in update_meta_value etc #2801

Currently when you update a meta value, it checks current_user_can('edit_post_meta') regardless of the actual object type. It needs to use object type dependent capability checks. These aren't in core yet but there's a ticket here:
https://core.trac.wordpress.org/ticket/38284

A workaround for the time being would be to check edit_{$object_type} I suppose.