Set the status on the existing action hooks

Rather than tracking down every place that might call
Instant_Articles_Post::to_instant_article and wrapping it call to set
the is_instant_article conditional, it's much simpler to just hook those
settings on the existing actions `instant_articles_before_transform_post`
and `instant_articles_after_transform_post`.

class-instant-articles-publisher.php

@@ -62,12 +62,8 @@ public static function submit_article( $post_id, $post ) {
 			return;
 		}
 
-		is_instant_article( true );
-
 		$article = $adapter->to_instant_article();
 
-		is_instant_article( false );
-
 		// Skip empty articles or articles missing title.
 		// This is important because the save_post action is also triggered by bulk updates, but in this case
 		// WordPress does not load the content field from DB for performance reasons. In this case, articles

facebook-instant-articles.php

@@ -151,12 +151,10 @@ function instant_articles_init() {
 	 * @since 0.1
 	 */
 	function instant_articles_feed() {
-		is_instant_article( true );
 
 		// Load the feed template.
 		include( dirname( __FILE__ ) . '/feed-template.php' );
 
-		is_instant_article( false );
 	}
 
 	/**
@@ -174,6 +172,8 @@ function is_instant_article( $set_status = null ) {
 
 		return $is_instant_article;
 	}
+	add_action( 'instant_articles_before_transform_post', function() { is_instant_article( true ); } );
+	add_action( 'instant_articles_after_transform_post', function() { is_instant_article( false ); } );
 
 	/**
 	 * Modify the main query for our feed.