Restore sitemap validation in unit test testLastModNow()

The unit test now longer fails when using my fork of SitemapGen4j (fork-1.1.0)
instead of the original version (1.0.1).

I updated the official schema siteindex.xsd from version 2006-07-25
to 2009-04-08 which updates and fixes the definition of lastmod dates.

See details:
https://github.com/eric-brechemier/sitemapgen4j/commit/e7bbe12