Why HTTP header response can be updated in a called function in Go?

I am learning Go and have a quick question about http handler implementation in Go. I am asking it in a small sample code.

So assume there is a handler function called Test() as defined like below

      func Test() func(http.ResponseWriter, *http.Request) {
        return func(w http.ResponseWriter, r *http.Request) {
            params := r.URL.Query()
            name := params.Get("name")
            if name == "axy" {
                common.UpdateHttpResponse("Trying to updating the response", w, http.StatusBadRequest)
                //return
            }
            w.WriteHeader(http.StatusOK)
        }
    }

lets say the UpdateHttpResponse method is also define in common package as below:

func LogExtractionErrorResponse(errMsg string, w http.ResponseWriter, responseHeader int) {
    fmt.Printf("%s", errMsg)
    jsonErrorOut := map[string]string{
        "Error": errMsg,
    }
    w.WriteHeader(responseHeader)
    encodedResponse, _ := json.Marshal(jsonErrorOut)
    if w != nil {   
        w.Write(encodedResponse)
    }
}

I call the Test() HTTP handler in the http server part as below.

// this is how Test() http handler is called as well
          http.HandleFunc("/test", httpserver.Test())

So here is my question:

• Based on my understanding all values are passed by value in go (as discussed in this thread as well)
• In that case why if the http handler is called with a parameter (i.e localhost:PORT_NUM/test?name=axy), i observe "StatusBadRequest" in resonse. In other words, why the commented "return" keyword is not needed and why the header response is not overwritten by "w.WriteHeader(http.StatusOK)" at the end of Test() http handler?