[bug]hyper attach return empty string

[YaoZengzeng]

At first , I run a container "hypercli run -itd busybox cat", and then attach it , input "hello\n", in test case TestAttachDetachTruncatedID(docker_cli_attach_unix_test.go)，it read an empty string and err = EOF.But if I do all of it by hand ,everything seems OK , its so confusing.And delay is useless