Update session.php

I made a slight change to the session.php destroy function because using this functions destroys all session set on browser including the template_path set in the config area.

So, I made the function to unset session by key alone
                     public static function destroy($key){

		                 if(self::$_sessionStarted == true){
			                unset($_SESSION[SESSION_PREFIX.$key]);
		                   }

	        }