Added forced reduction to Date object

Hi.

I use Mac OS and I have some issues. 

My meta-data for index.hmtl

```
---
title: ''
layout: 'default'
dynamic: true
standalone: true
---
```

When I run `docpad server --env ru` files with css and javascript not loaded, because I see 500 error. 

In console I have this: `TypeError: Object 2014-01-25T14:49:35.294Z has no method 'toUTCString'`
In Headers on page I saw that Etag have not valid value.

Response Headers

```
Connection:keep-alive
Content-Length:3858
Content-Type:text/html
Date:Sat, 25 Jan 2014 12:56:27 GMT
ETag:"5226-NaN"
X-Powered-By:Express, DocPad v6.60.3
```

It fix make Date object from mtime date always.