wroteChunk
Handles the #SoupMessageBody part of writing a chunk of data to the network.
Normally this is a no-op, but if you have set @body's accumulate flag to false, then this will cause @chunk to be discarded to free up memory.
This is a low-level method which you should not need to use, and there are further restrictions on its proper use which are not documented here.
Parameters
chunk
a #GBytes returned from method@MessageBody.get_chunk