Merge pull request #263 from jasperrooduijn/php82

Match jsonSerializable interface.

src/WordPress_Object/Clarkson_Object.php

@@ -626,7 +626,7 @@ public function get_comments( array $args = array() ): array {
 	 *
 	 * We can't just return $this->_post, because these values will only return raw unfiltered data.
 	 */
-	public function jsonSerialize() {
+	public function jsonSerialize(): mixed {
 		$data              = array();
 		$data['id']        = $this->get_id();
 		$data['link']      = $this->get_permalink();