Do not write HTTP headers with SkipResponseBodyEncodeDecode

[raphael]

Make it possible for user code to write HTTP headers.

Prior to this fix using SkipResponseBodyEncodeDecode would generate the following encoder code:

// EncodeStreamResponse returns an encoder for responses returned by the events
// stream endpoint.
func EncodeStreamResponse(encoder func(context.Context, http.ResponseWriter) goahttp.Encoder) func(context.Context, http.ResponseWriter, any) error {
	return func(ctx context.Context, w http.ResponseWriter, v any) error {
		w.WriteHeader(http.StatusOK)
		return nil
	}
}

With this change the function is now:

// EncodeStreamResponse returns an encoder for responses returned by the events
// stream endpoint.
func EncodeStreamResponse(encoder func(context.Context, http.ResponseWriter) goahttp.Encoder) func(context.Context, http.ResponseWriter, any) error {
	return func(ctx context.Context, w http.ResponseWriter, v any) error {
		return nil
	}
}

This makes it possible for user code to write custom headers, for example to support SSE.