Check Front-End-Https header for __SECURE__ env

Check the Front-End-Https header when determining whether or not the
connection is secure to allow Symphony to exist behind an HTTPS
downstream proxy.
Make sure file exists.
Change to X header

Change the downstream-HTTPS switch to X-Forwarded-Proto
Change value of X header that indicated https

It should be 'https', not 'on'... Swift.
USe $_SERVER instead of apache function

^^
Check for X-Forwarded-Proto header in a more succinct fashion

This change is the result of feedback from pull request 1498 to
symphonycms/symphony-2:
    https://github.com/symphonycms/symphony-2/pull/1498
Merge pull request #1505 from Sidevices/master

Use X-Forwarded-Proto header to detect secure connections
Merge pull request #1501 from vlad-ghita/patch-1

Make sure file exists in the error handler.
Release 2.3.1RC3
Fixed typo in HTMLpage->removeFromHead()
Fix ResourceManager regular expression

This allows Symphony to be installed in a folder like `extensions/`.
Wrap querystring in CDATA tags
Fix phpdoc to point to the main php.net website
Merge branch 'integration' of https://github.com/symphonycms/symphony-2 into nitriques-integration
Check $about['handle'] isset
Remove the LIMIT clause in the Cache driver. Closes #1517
Merge pull request #1515 from designermonkey/cdata-qstring

Wrap querystring in CDATA tags. Closes #1480
Release 2.3.1
Corrected date format for entry creation
Merge pull request #1520 from designermonkey/datetime

Corrected date format for entry creation. Closes #1518