Updated docs to reflect that Response.change(body: null) will not work

[jonasfj]

Passing null into Response.change(body: null) does not create a new Response with no body as the documentation would have one think.

Passing an empty list should create the same result as passing null.

I did consider fixing the code as follows, but I think that might be a bit too fancy :)

  @override
  Response change({
    Map<String, String> headers,
    Map<String, Object> context,
    body = #_notChanged,
  }) {
    headers = updateMap(this.headers, headers);
    context = updateMap(this.context, context);

    if (body == #_notChanged) {
      body = extractBody(this);
    }

    return Response(statusCode, body: body, headers: headers, context: context);
  }