API push should be called on action hook 'publish_post', not 'save_post'

[eobanb]

Issue: post data can be incorrect in certain circumstances
Cause: save_post action is used for npr_push()
Suggested fix: use publish_post instead

In debugging a problem with audio not being attached to our pushed posts, we discovered an incompatibility between WP-DS-NPR-API and a Wordpress Mu plugin we use, Sitewide Tags. The $post object used in post_to_nprml_story() corresponded to a copy of the original post created by the Sitewide Tags plugin.

This led me to notice that the add_action() function in the plugin file push_story.php that calls npr_push() is attached to Wordpress action hook save_post. This caused a problem, because under some conditions it's possible for save_post to return post data from an unintended post, such as a post revision (or, in this case, the reference copy of the post made by Sitewide Tags).

While save_post does usually work, the better action to use in this case is publish_post, which is a very similar—but not identical—action; publish_post always returns the actual post's data, and only when its transition status goes to 'publish'.

(side note: the decision to use save_post when this plugin was written likely arose because publish_post was previously mistakenly marked as deprecated in the Wordpress API reference)