Change the implementation of SimpleSAML\Utils\HTTP::getSelfURL() and...

Change the implementation of SimpleSAML\Utils\HTTP::getSelfURL() and getSelfURLNoQuery() to honor the 'baseurlpath' configuration option instead of simply using the environment. They were actually broken since they were using it to build the scheme, host and port, but completely ignoring the path, rendering wrong URLs in between what was configured in 'baseurlpath' and the real information in the environment. This resolves #396, but also affects #5. The changes to getSelfURLNoQuery() in #391 are unnecessary now, since we now basically getting the full URL and remove the query afterwards.