[2.1.x] Fixed CVE-2019-12781 -- Made HttpRequest always trust SECURE_PROXY_SSL_HEADER if set.

      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.

Backport of 54d0f5e62f54c29a12dd96f44bacd810cbe03ac8 from master