Handle CGI SSL detection via HTTPS environment better.

Some servers send on/off, 0/1, yes/no, and may be upper or lower case.
This fix will handle all of those permutations. It was inspired by the
detection done in in some other wsgi python web applications I looked at.

mercurial/hgweb/wsgicgi.py

@@ -23,7 +23,7 b' def launch(application):'
     environ['wsgi.multiprocess'] = True
     environ['wsgi.run_once'] = True
 
-    if environ.get('HTTPS','off') in ('on','1'):
+    if environ.get('HTTPS','off').lower() in ('on','1','yes'):
         environ['wsgi.url_scheme'] = 'https'
     else:
         environ['wsgi.url_scheme'] = 'http'