Add verification of parsing results in Event.

Prevent the server from crashing when someone passes valid, but
wrongly formatted JSON.

Sending a bad JSON string to the server can bring it down with a
NoMethodError exception. For instance:
<code>encoded_data == "[[\"websocket_rails.pong\", {}]]"</code>

Added the spec to test for the fix. It fails without the fix, but passes after.
I think it should be enough. :D