container_test: change ExportContainer test on unix socket
container_test: change ExportContainer test on unix socket

Reading the request before writing the response to make it behave like a
real server. Should fix the test on Linux and the flakiness on OS X.

csrwng/go-dockerclient,ybogdanov/go-dockerclient,fsouza/go-dockerclient,tarsisazevedo/go-dockerclient,ybogdanov/go-dockerclient,paulbellamy/go-dockerclient,davidstack/go-dockerclient,davidstack/go-dockerclient,csrwng/go-dockerclient,tarsisazevedo/go-dockerclient