Sometimes the django application is deployed behind a reverse-proxy
which manage HTTPS. In such case, django think the connection is plain
HTTP unless SECURE_PROXY_SSL_HEADER is set and the reverse-proxy sets
the appropriate header/value.
Simply using bool() on the string returns True for every non-empty
string. Hence, the only way to set a value to False is to set it the an
empty string, which is counter-intuitive.
This commit will instead compare the string to well-known values for
True and return accordingly. Every other non-empty string is evaluated
to False. Empty string are evaluated to the default value.
