Use https with X-Forwarded-Proto

[daliborjanak]

We need to change protocol detection. Is server have https only via CloudFlare than it retturns http in urls. Cloudflare give there a header "X-Forwarded-Proto: https"
add to line https://github.com/klokantech/tileserver-php/blob/master/tileserver.php#L1231 something like

or $_SERVER['X-Forwarded-Proto'] == 'https'

[daliborjanak]

Implemented