* Sink.queue and Sink.actorRefWithAck can now handle one element input
buffers
* Sink.queue now buffers as much upstream elements as declared in its
InputBuffer attribute, instead of one less.
* Tests for streaming with full buffer now do not depend on default
input buffer size (16); instead, they set sink input buffer size
explicitly.
* Sink.{queue, actorRefWithAck} and StreamConverters.{asInputStream,
asOutputStream} now use overriden/inherited `InputBuffer` attribute
* They now use their default attributes as initial attributes.
