Merge pull request #560 in ZP/z-push from...

Merge pull request #560 in ZP/z-push from bugfix/ZP-1256-z-push-admin-a-resync-t-folderid to develop

* commit 'd9b63fdd':
  ZP-1256 Set backend2folderidCache of ASDevice to false, so it can be rebuilt.
  ZP-1256 Rebuild backend2folderidCache of ASDevice if it's not available.
  ZP-1256 Rebuild backend2folderidCache of ASDevice if it's not available.

src/lib/core/asdevice.php

@@ -188,7 +188,7 @@ class ASDevice extends StateObject {
         unset($this->forceSave);
         unset($this->newdevice);
         unset($this->ignoredMessageIds);
-        unset($this->backend2folderidCache);
+        $this->backend2folderidCache = false;
 
         if (isset($this->ignoredmessages) && is_array($this->ignoredmessages)) {
             $imessages = $this->ignoredmessages;