handling header.Expires Date since we can't set current Date from json config

Example
```json
"static": {
    "module": "mixdown-plugins#Static",
    "options": {
        "headers": {
            "Pragma": "public",
            "Cache-Control": "max-age=2592000, public",
            "Expires": "2592000000"
        }
    }
}
```