Fix snippet selection

The webspaceKey attribute is null in the previewer, but the key is available in the webspace attribute.

This fixes #51.

Document/Structure/ContentProxyFactory.php

@@ -150,6 +150,6 @@ private function getWebspaceKey(): ?string
             return null;
         }
 
-        return $attributes->getAttribute('webspaceKey');
+        return $attributes->getAttribute('webspaceKey') ?? $attributes->getAttribute('webspace')->getKey();
     }
 }