API Fix HTTPS proxy header detection

Didn't use the de facto standard HTTP_X_FORWARDED_PROTO or the less standard HTTP_FRONT_END_HTTPS.
Removed the 'X-Forwarded-Proto', since PHP should prefix/underscore all HTTP headers before it hits $_SERVER.

References:
- https://docs.djangoproject.com/en/1.4/ref/settings/#secure-proxy-ssl-header
- https://drupal.org/node/1859252
- https://drupal.org/node/313145
- http://scottwb.com/blog/2013/02/06/always-on-https-with-rails-behind-an-elb/
Merge pull request #3152 from silverstripe-iterators/pulls/https-proto

API Fix HTTPS proxy header detection