The main question: is it guaranteed that Write method of http.ResponseWriter either returns non-nil error or writes all the data?

If not, then it should be encouraged to use io.Copy(rw, bytes.NewReader(...)). Somehow I'm sure, that a lot of code already relies on full writes and my proposed construction isn't elegant for such boilerplate code

If yes, I would propose to add the guarantee to the comment for Write method

I believe that the guarantee should be provided, because the alternative is unobvious and adds unnecessary boilerplate, opposed to the maintenance of the property