Fix timezone in sitemap.xml

`date:"%Y-%m-%dT%H:%M:%SZ"` doesn't convert the date/time to UTC and outputs the date/time in the original timezone.

`date_to_xmlschema` is used for other dates in the same file and outputs date/times correctly.

```
Before: 2014-09-21T17:28:15Z
After:  2014-09-21T17:28:15+05:30
```