Cannot change session id when session is active

mezzio#26
Fix provided by https://github.com/jorgeribeiro

Signed-off-by: Timo Nußbaum <[email protected]>

Author: Inchie

src/PhpSessionPersistence.php

@@ -167,7 +167,10 @@ public function initializeId(SessionInterface $session): SessionInterface
      */
     private function startSession(string $id, array $options = []): void
     {
-        session_id($id);
+        if (!session_id()) { // Check if session is already set beforehand
+            session_id($id);
+        }
+        
         session_start([
             'use_cookies'      => false,
             'use_only_cookies' => true,