add x-forwarded-proto as secure scheme default

Heroku uses this and it's the only one mentioned in Wikipedia's
list of common non-standard headers.

http://en.wikipedia.org/wiki/List_of_HTTP_header_fields

gunicorn/config.py

@@ -668,6 +668,7 @@ class SecureSchemeHeader(Setting):
     validator = validate_dict
     default = {
         "X-FORWARDED-PROTOCOL": "ssl",
+        "X-FORWARDED-PROTO": "https",
         "X-FORWARDED-SSL": "on"
     }
     desc = """\