Fix timezone in sitemap.xml

`date:"%Y-%m-%dT%H:%M:%SZ"` doesn't convert the date to UTC and outputs in the original timezone but says `Z` at the end which makes search engines think it's in UTC.

`date_to_xmlschema` is used for other dates in the same file and outputs dates without issue.

```
Before: 2014-09-21T17:28:15Z
After:  2014-09-21T17:28:15+05:30
```