Unset tags relation after setting (#131)

This ensures that the proper tag values are returned to the API by clearing any cached tags before returning a response. It also makes sure that the listeners to the `DiscussionWasTagged` event won't have old data under `$event->discussion->tags`.

Fixes https://github.com/flarum/core/issues/2514

extensions/tags/src/Listener/SaveTagsToDatabase.php

@@ -118,6 +118,7 @@ class SaveTagsToDatabase
 
             $discussion->afterSave(function ($discussion) use ($newTagIds) {
                 $discussion->tags()->sync($newTagIds);
+                $discussion->unsetRelation('tags');
             });
         }
     }