Thumbnails for post types that does'nt support and docs update

The hook "plugins_loaded" does'nt work with the "json_endpoints". It would be good to update the docs:
https://github.com/phh/WP-API/blob/b2471aa355a84eb180ca7ab423bd59eecf236a88/docs/guides/extending.md#registering-your-endpoints

The json file always contains "featured_image: null" even for my post types that does'nt even support thumbnails. Checking for "post_type_supports( $post['post_type'], 'thumbnail' )" before appending that data to the json file would be a good idea.