This patch fixes a bug that prevents sent data from being flushed to the
socket.
When data is sent, the task managing the connection must be notified. A
guard exists that prevents unnecessary notification of the connection
when the stream does not have any send capacity. However, this guard was
buggy. Instead of notifying the connection if *any* data can be sent, it
notified the connection only when *all* data could be sent.
This patch fixes the check as well as adds some tests that ensure the
connection task is notified.
5d54d8cd79