akka/akka — pull request #16267 by drewhk (2014-11-10 14:31:19+00:00)
Title: =act #15991 delay sending of ACK until .. (forward port)

Otherwise, the user-level actor may already have sent the next chunk before
the pendingWrite has been updated and the TcpConnection will reject it.
(cherry picked from commit 96758e6)