Fixed #7603 -- Added a 'scheme' property to the HttpRequest object

HttpRequest.scheme uses self.is_secure() to determine if scheme is
'http' or 'https' wether WSGIRequest.scheme makes use of the
'wsgi.url_scheme' WSGI environ variable.

This provides a handful method to check for current scheme in, for
example, templates. It also allows to deal with other schemes.