improve io.Discard for defer

[kamilsk]

if I use io.Discard with defer I have bad dev experience:

resp, err := http.Do(req)
if err {...}
defer safe.Close(io.Discard(resp.Body), ...) // now I can't read from body

json.NewDecoder(resp.Body).Decode() // woops

[kamilsk]

and write docs with tests