RestPubSub does not work with ByteArrayPubSubMessageSerDe

[akshaisarma]

When we write out the PubSubMessage (created with a SerDe), the RestPubSub takes the payload (which is a byte[]), serializes it again to another byte[], Base64 encodes it as a String and returns a new PubSubMessage that is then JSONified. When reading it back in RestSubscriber, we are not able to GSON parse it as a PubSubMessage since it expects an array but a String is there instead: Expected BEGIN_ARRAY but was STRING