An HTTP request would not be redirected to HTTPS when the
SECURE_PROXY_SSL_HEADER and SECURE_SSL_REDIRECT settings were used if
the proxy connected to Django via HTTPS.
HttpRequest.scheme will now always trust the SECURE_PROXY_SSL_HEADER if
set, rather than falling back to the request scheme when the
SECURE_PROXY_SSL_HEADER did not have the secure value.
Thanks to Gavin Wahl for the report and initial patch suggestion, and
Shai Berger for review.
