Method for checking $_SERVER['HTTPS'] not compatible with nginx/1.1.19

I grepped through trunk with "grep -rn SERVER.*HTTPS ." and found a couple of instances in CRM/Core/Config.php and one in CRM/Utils/ReCAPTCHA.php using the !='on' formula which I changed to =='off'.

I also found a few with =='on' (e.g.CRM/Utils/System.php) and left these as is.

There were a few other instances in packages with the !='off' which I imagine would not be "nginx safe" but this worked for me.

I'm not sure if this will break non nginx sites or why we used !='off' rather than =='on' in the first place. FWIW I also noticed that some of them used " === 'on' " in case someone thinks that would be more bullet proof.