Don't clear buffer when consuming stream

The `cork_buffer_to_stream_consumer` function claims that the resulting
stream consumer will append to the buffer, but previously it would clear
the buffer when it received the first chunk of data.  That means that we
could't consume data into an existing buffer while keeping any previous
contents.  This patch updates the consumer implementation to actually
perform an append.
Merge branch 'feature/append-buffer' into develop

* feature/append-buffer:
  Don't clear buffer when consuming stream