Fix that updating INI repositories is impossible

(cherry picked from commit d5de7e9)
Signed-off-by: Eric Lippmann <eric.lippmann@icinga.com>

library/Icinga/Repository/IniRepository.php

@@ -302,7 +302,9 @@ public function update($target, array $data, Filter $filter = null)
             $newConfig = clone $config;
             foreach ($newData as $column => $value) {
                 if ($column === $keyColumn) {
-                    $newSection = $value;
+                    if ($value !== $config->get($keyColumn)) {
+                        $newSection = $value;
+                    }
                 } else {
                     $newConfig->$column = $value;
                 }