Base64 encode protocol buffer payload prior to sending message to IronMQ

After some testing, we noticed that our protocol buffer payload is sometimes modified in between sending the message to IronMQ and receiving the message from IronMQ.

From the IronMQ documentation, it also appears as though the message payload is expected to be json. The new payload pushed from the IronMQ published and expected by the subscriber is like such:

```json
{"payload" :  Base64 Encoded Protocol Buffer }
```