Adding payload to the Http2Client example.

Modifications:
When trying out the Http2Client example I noticed that adding a request
payload would not cause a data frame to written. This seems to be
because writeHeaders completes the promise, and then the writeData
call ends up in FlowControlWriter.writeFrame, isDone is true and
the data released and the call aborted and returned.
Adding a new promise for the writeData method allows a data frame to be
written.

Result:
A body/payload can now be sent to the server. The example was updated to
simply echo the payload received back to the calling client.
Ok... then please fix the comment to let it say "POST" and not GET ;)