Fix the category_slugs format in the Patterns API (#711)

This fixes a regression caused via #704 that returns category_slugs as an object instead of an array

public_html/wp-content/plugins/pattern-directory/includes/pattern-post-type.php

@@ -295,6 +295,7 @@ function register_rest_fields() {
 				$slugs = wp_list_pluck( wp_get_object_terms( get_the_ID(), 'wporg-pattern-category' ), 'slug' );
 				$slugs = array_map( 'sanitize_title', $slugs );
 				$slugs = array_diff( $slugs, [ 'featured' ] );
+				$slugs = array_values( $slugs );
 
 				return $slugs;
 			},