Don't base64 decode data in CloudPubSubSource

[nachocano]

Problem
Currently, if the data is a json, we are unmarshalling it, and sending the raw data. We do not do this for other types, e.g., xml... We agreed not to unmarshall the payload, and just send the data as is (base64 encoded)

Persona:
Which persona is this feature for?
Developer

Exit Criteria
Not base64 decode json in CloudPubSubSource. Add UTs

Time Estimate (optional):
< 1h

Additional context (optional)
Always do this https://github.com/google/knative-gcp/blob/master/pkg/pubsub/adapter/converters/pubsub.go#L72, there is no need for the if statement.

[nachocano]

/assign @capri-xiyue

[bskaplan]

/assign