Optional ETag and Content-Format in SetResponse

[illmade-knight]

The ResponseWriter by default adds options for ETag and Content-Format if a body io.ReadSeeker is present.
We're very byte conscious and as neither are required by RFC 7252 it would be preferable if their inclusion was optional.

[niondir]

I also like the idea and it seems straight forward to implement.

How should the API look like?Just give access to the response object of the response writer?

[niondir]

You can already send a response without ETag and ContentType when using the Client()

The only remaining side-effect is, that an ACK is send for Confirmable requests and you can not piggyback your response this way.

	m.Handle("/a", mux.HandlerFunc(func(w mux.ResponseWriter, r *mux.Message) {
		assert.Equal(t, codes.GET, r.Code)
		customResp := message.Message{
			Code:    codes.Content,
			Token:   r.Token,
			Context: r.Context,
		}

		err = w.Client().WriteMessage(&customResp)
		if err != nil {
			log.Printf("cannot set response: %v", err)
		}
	}))

[illmade-knight]

I'll need to dig back into this but I think the Confirmable request was actually the exact problem I was having where I needed to use w.SetResponse(...) instead of w.Client().WriteMessage(...)
In the meantime I've just set a minimal ETag in SetResponse's options to keep the bytes down

[JosefWN]

Related to: #349

[jkralik]

Implemented in v3. Feel free to reopen it. Thx