Fix CopyFromContainer test that wasn't testing its output stream.
Fix CopyFromContainer test that wasn't testing its output stream.

We were basically comparing 2 test variables instead of actually
checking the output stream contents.

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