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

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

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

`HttpRequest.is_secure()` simply checks if `HttpRequest.scheme` is
`https`.

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

Thanks nslater for the suggestion.