Session: used static access to $started (#104)

Deprecated error was triggered.
$this->started instead of self::$started

src/Http/Session.php

@@ -407,7 +407,7 @@ private function configure(array $config)
 
 			} else {
 				if (session_status() === PHP_SESSION_ACTIVE) {
-					throw new Nette\InvalidStateException("Unable to set 'session.$key' to value '$value' when session has been started" . ($this->started ? '.' : ' by session.auto_start or session_start().'));
+					throw new Nette\InvalidStateException("Unable to set 'session.$key' to value '$value' when session has been started" . (self::$started ? '.' : ' by session.auto_start or session_start().'));
 				}
 				if (isset($special[$key])) {
 					$key = "session_$key";