testing: fix reading body in fake server's attach
testing: fix reading body in fake server's attach

liron-l/go-dockerclient,swagiaal/go-dockerclient,cloudimmunity/go-dockerclientx,paulbellamy/go-dockerclient,achanda/go-dockerclient,eliasgs/go-dockerclient,ybogdanov/go-dockerclient,bboreham/go-dockerclient,davidstack/go-dockerclient,jefferai/go-dockerclient,codekitchen/go-dockerclient,tarsisazevedo/go-dockerclient,lindenlab/go-dockerclient,csrwng/go-dockerclient,flowlo/go-dockerclient,tarsisazevedo/go-dockerclient,fsouza/go-dockerclient,davidstack/go-dockerclient,csrwng/go-dockerclient,ybogdanov/go-dockerclient