Evaluate the env 'HTTPS' without a case.

Environment of My Service(Using HTTPS)
- ServerEnv HTTPS's value is 'ON' (not 'on').
- Port of https is not 443/tcp.

I expect to get 'https' from Facebook::getHttpProtocol() .
But in this case, Facebook::getHttpProtocol() returns 'http'.

So I changed the condition to evaluate $_SERVER['HTTPS'].