Url::get_absolute_url(): fix checking if the public root URL is relative

src/Auth/User.php

@@ -138,9 +138,13 @@ class User {
       Log::debug("%s->save(): no change to save", $this);
       return true;
     }
+    $changes = [];
+    foreach ($this->info as $attr => $value)
+      if ($this->original_user->$attr != $value)
+        $changes[$attr] = $value;
     return call_user_func(
       array($this -> backend, 'update_user'),
-      $this -> original_user, $this -> info, true
+      $this -> original_user, $changes, true
     );
   }