QNetworkDiskCache: Do not store set-cookie headers of the response in disk cache.

The cookies may contain sensitive information. so we should not store cookies in disk cache.

Task-number: QTBUG-42546
Change-Id: I6331bdd766445af41f55bfaf0e9132b75dd7957f
Reviewed-by: Jeongmin Kim <jm86.kim@lge.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>