[stable5] Fixes for update feed fetching and parsing

The update feed never expires as the current time is stored into the settings right before the expiration check.
In addition there was a false positive while checking OC_Updater::check result.

Closes https://github.com/owncloud/core/issues/4824
https://github.com/owncloud/core/issues/5260#issuecomment-26060984