PubSubIO readMessagesWithMessageId() breaks the payload encoding when using DataflowRunner

My pipeline reads PubSub messages and parses their payload to objects using Gson. I use PubsubIO.readMessagesWithMessageId() to get the PubSub message and the message ID.

I tested the pipeline thoroughly by running it with the DirectRunner in my local machine and everything works fine, but when running it as a Dataflow job in GCP using the DataflowRunner, Gson can't parse the messages properly because the first character of the payload (opening bracket "{") is missing, this only happens when using the DataflowRunner. 

I noticed that the problem no longer happens when using PubsubIO.readStrings() instead of PubsubIO.readMessagesWithMessageId() and getting the payload directly (previously I had to decode the payload using new String(element.getPayload(), StandardCharsets.UTF_8); )

Imported from Jira [BEAM-10113](https://issues.apache.org/jira/browse/BEAM-10113). Original Jira may contain additional context.
Reported by: alexandermalyga.