[Processor/GCP - PubSub] Avro event re-format when pushed to PubSub

Hi, 

as discussed on Slack : https://apachenifi.slack.com/archives/C0L9VCD47/p1596463216211700

When we generate an avro event from record and we publish it using PublishPubSub processor the content-type of the file change from avro/binary to UTF-8 due to the following code part : https://github.com/apache/nifi/blob/main/nifi-nar-bundles/nifi-gcp-bundle/nifi-gcp-processors/src/main/java/org/apache/nifi/processors/gcp/pubsub/PublishGCPubSub.java#L157

This make the event unreadable once Consumed :/ 

The workaround we found is base64 encoding the avro message, publishing it to PubSub, consuming it, and finally decoding it. 

We expect this part of encoding would be handled by the processors.