Fix bug in engine.Sender

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
engine/spawn: run an engine in a subprocess, remote-controlled by Beam

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)
Merge pull request #5422 from shykes/engine-spawn

engine/spawn: run an engine in a subprocess, remote-controlled by Beam