Broken login redirect when transparently hiding the webapp path through a reverse proxy

Scenario:

• deploy XWiki under a name other than the empty one (ROOT), for example xwiki
• use xwiki.webapppath=/ in xwiki.cfg
• configure httpd as a reverse proxy passing http://hostname/ to ajp://localhost:8080/xwiki/
• while not logged in, access a URL that requires authentication
• the browser is redirected to http://hostname/xwikibin/login/XWiki/XWikiLogin (note the missing slash between xwiki and bin)