RFC3339 compliant updated field (spatie#136)

Author: olipayne

src/FeedItem.php

@@ -62,7 +62,7 @@ public function title(string $title): self
 
     public function updated(Carbon $updated): self
     {
-        $this->updated = $updated;
+        $this->updated = $updated->toRfc3339String();
 
         return $this;
     }