Fix Firefox 32

Event objects passed into page_worker message handlers now appear to be immutable in FF32.  We were mutating the event object by adding a setResponse function and replacing _callback_x strings in the message args with onSuccess and onError handlers.  This is why we were seeing exceptions about onError being undefined.  Making a deep copy of the message data fixes the issue.

It looks like events passed in contexts other than page_worker are still mutable, but we should still fix them to protect against changes like this in the future.  Mutating input parameters is a bad idea, especially when those parameters come from third-party code.