make // absolute urls without scheme work for sitemaps

Solving issue #965 , however was not able to test. Please tell me how to run test files in current scrapy version (docs say "bin/runtests.sh", but this file does not longer exist).

Secondly, this only solves the reported issue with `SitemapSpider`. Yet, the problem exists for all spiders. `Request(referrer=)` has to be set in all other spiders, too.