Allow to force IE style headers for attachments in non-HTTPS session

[Pecisk]

I suggest to introduce new configuration variable which would force the usage of headers now used specially for attachments while using Internet Explorer and HTTPS. Code in question is in program/include/rcube_share.inc:

if (isset($_SERVER['HTTPS'])) {
   header('Pragma: ');
   header('Cache-Control: ')
}

This is necesary for solutions where PHP code is parsed on non-HTTPS daemon and then delivered trough HTTPS connection to client (Combination of lightttpd and Apache comes into main).

This would work like this: having configuration variable set to true would force to pass such headers to client while providing attachment. "If" block mentioned above just need additional check and vola - it's set.

[alec]

Added 'use_https' option in r2927.