Change the serialization of date

Before this, I had the following silent error : 
```javascript
[{"error":{"code":111,"error":"invalid type for key completedAt, expected date, but got string"}}]
```

According to this link, https://www.parse.com/questions/which-json-date-format-is-supported-by-rest-api, we should serialize a date like that : 
```javascript
{
  "__type": "Date",
  "iso": "2013-07-09T08:43:12.000Z"
}
```