Fix < loc > URLs generation for URLs with '&'

see https://github.com/osclass/plugin-sitemap_generator/pull/2 

To Convert special characters, especially the **&** found in OSClass search URLs, to HTML entities

taken from https://github.com/pawelantczak/php-sitemap-generator/blob/master/SitemapGenerator.php#L190 an excellent sitemap generator
Merge pull request #4 from emanwebdev/patch-1

Fix < loc > URLs generation for URLs with '&'