Preparing meta using get_post_meta

Maybe it's better to use get_post_meta instead has_meta, as this way we make use of the caching system. And i don't think the meta id is relevant in any way
Update class-wp-json-posts.php
Update class-wp-json-posts.php
Update class-wp-json-posts.php
Merge pull request #14 from afurculita/patch-1

Preparing meta using get_post_meta