fixes lazy-loaded session (method name changed in Symfony)

src/Silex/Extension/SessionExtension.php

@@ -47,7 +47,7 @@ class SessionExtension implements ExtensionInterface
         $request->setSession($this->app['session']);
 
         // starts the session if a session cookie already exists in the request...
-        if ($request->hasSession()) {
+        if ($request->hasPreviousSession()) {
             $request->getSession()->start();
         }
     }