Ensure that the "Messages" portion of a PersistentResponse is an array

- .NET client
- Previously it was possible to iterate over an object instead
- Thanks to @JamesNK for pointing it out

https://github.com/SignalR/SignalR/commit/c20d822b46d60bdf2003e39c9c2e7b6a58873bf8#commitcomment-4729636

#2691