Make hg serve set the wsgi.url_scheme property correctly.
This fixes the bug where hg serve with SSL will always detect the wrong
urlbase for templates. This is also part of unifying the CGI interface
and hg serve interface to both use standard wsgi.
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.