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

`HttpRequest.scheme` is `https` if `settings.SECURE_PROXY_SSL_HEADER`
appropriately set, falls back to `HttpRequest._get_scheme()` (a hook for
subclasses to implement) otherwise.

`WSGIRequest._get_scheme()` makes use of `wsgi.url_scheme` WSGI environ variable
to determine the request scheme.

`HttpRequest.is_secure()` not simply checks if `HttpRequest.scheme` is `https`.

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