You're dealing with a chunked response. I'm not sure what your end goal is but there are a few different options. The source itself says;
// Body represents the response body. // // The http Client and Transport guarantee that Body is always // non-nil, even on responses without a body or responses with // a zero-length body. It is the caller's responsibility to // close Body. // // The Body is automatically dechunked if the server replied // with a "chunked" Transfer-Encoding. Body io.ReadCloser
So for example here;
response, err := ioutil.ReadAll(resp.Body) where you're relaying the response from the other service, you could fix the problem by making the service that provided
resp set a Transfer-Encoding header with the value chunked, assuming you have access to that api as well. If you're only working in this middle layer, then you'll have to dechunk the response yourself before writing it. If the request you're monitoring in Fiddler doesn't have
chunked Transfer-Encoding, just adding that may cause Fiddler to display it the same as you see it in Postman.