Conversation.cloneMessage throws away Symbol and Date objects

[drhayes]

In working with the botkit I was trying to pass some custom middleware some Symbols on the message object. By the time the middleware gets them, the Symbols have all been nulled out.

Turns out that's because the cloneMessage method on Conversation uses JSON.parse and JSON.stringify to deep clone the message object. This method is great and really fast, except it discards values that aren't JSON-compatible, like Dates and Symbols and Functions. Not cool, man, not cool.