Http1Response uses a PushPublisher to control the byte buffer flow.
However, the PushPublisher makes assumptions that are not always true:

 - The PushPublisher assumes it runs in a single thread: this is only
   guaranteed if the request is sent in synchronous mode.

 - The PushPublisher assumes that the consumer is always ready to
   accept new items. In asynchronous mode, the consumer may
   accept onNext in one thread, but emit Subscription.request() in
   another thread - and will expect the publisher to wait before sending
   the next item if it has not requested any.